Although any set up of cpusets can really be described as "shielding," there
is one prevalent shielding model in use that is so common that cset has a
subcommand that is dedicated to its use. This subcommand is called shield.
The concept behind this model is the use of three cpusets. The root cpuset
which is always present in all configurations and contains all CPUs. The
system cpuset which contains CPUs which are used for system tasks. These
are the normal tasks that are not "important," but which need to run on the
system. And finally, the user cpuset which contains CPUs which are used for
"important" tasks. The user cpuset is the shield. Only those tasks that
are somehow important, usually tasks whose performance determines the overall
rating for the machine, are run in the user cpuset.
The shield subcommand manages all of these cpusets and lets you define the
CPUs and Memory Nodes that are in the shielded and unshielded sets. The
subcommand automatically moves all movable tasks on the system into the
unshielded cpuset on shield activation, and back into the root cpuset on
shield tear down. The subcommand then lets you move tasks into and out of the
shield. Additionally, you can move special tasks (kernel threads) which
normally run in the root cpuset into the unshielded set so that your
shield will have even less disturbance.
The shield subcommand abstracts the management of these cpusets away from
you and provides options that drive how the shield is set up, which tasks are
to be shielded and which tasks are not, and status of the shield. In fact,
you need not be bothered with the naming of the required cpusets or even where
the cpuset filesystem is mounted. Cset and the shield subcommand takes
care of all that.
If you find yourself needing to define more cpusets for your application, then
it is likely that this simple shielding is not a rich enough model for you.
In this case, you should transition to using the set and proc subcommands
described in a later section.
2.1. A Simple Shielding Example
Assume that we have a 4-way machine that is not NUMA. This means there are 4
CPUs at our disposal and there is only one Memory Node available. On such
machines, we do not need to specify any memory node parameters to cset, it
sets up the only available memory node by default.
Usually, one wants to dedicate as many CPUs to the shield as possible and
leave a minimal set of CPUs for normal system processing. The reasoning for
this is because the performance of the important tasks will rule the
performance of the installation as a whole and these important tasks need as
many resources available to them as possible, exclusive of other, unimportant
tasks that are running on the system.
Note
|
I use the word "task" to represent either a process or a thread that is
running on the system. |
2.1.1. Setup and Teardown of the Shield
To set up a shield of 3 CPUs with 1 CPU left for low priority system
processing, issue the following command.
[zuul:cpuset-trunk]# cset shield -c 1-3
cset: --> activating shielding:
cset: moving 176 tasks from root into system cpuset...
[==================================================]%
cset: "system" cpuset of CPUSPEC(0) with 176 tasks running
cset: "user" cpuset of CPUSPEC(1-3) with 0 tasks running
This command does a number of things. First, a user cpuset is created with
what’s called a CPUSPEC (CPU specification) from the -c/--cpu option. This
CPUSPEC specifies to use CPUs 1 through 3 inclusively. Next, the command
creates a system cpuset with a CPUSPEC that is the inverse of the -c
option for the current machine. On this machine that cpuset will only contain
the first CPU, CPU0. Next, all userspace processes running in the root
cpuset are transfered to the system cpuset. This makes all those processes
run only on CPU0. The effect of this is that the shield consists of CPUs 1
through 3 and they are now idling.
Note that the command did not move the kernel threads that are running in the
root cpuset to the system cpuset. This is because you may want these
kernel threads to use all available CPUs. If you do not, the you can use the
-k/--kthread option as described below.
The shield setup command above outputs the information of which cpusets were
created and how many tasks are running on each. If you want to see the
current status of the shield again, issue this command:
[zuul:cpuset-trunk]# cset shield
cset: --> shielding system active with
cset: "system" cpuset of CPUSPEC(0) with 176 tasks running
cset: "user" cpuset of CPUSPEC(1-3) with 0 tasks running
Which shows us that the shield is set up and that 176 tasks are running in the
system cpuset—the "unshielded" cpuset.
It is important to move all possible tasks from the root cpuset to the
unshielded system cpuset because a task’s cpuset property is inherited by
its children. Since we’ve moved all running tasks (including init) to the
unshielded system cpuset, that means that any new tasks that are spawned
will also run in the unshielded system cpuset.
Some kernel threads can be moved into the unshielded system cpuset as well.
These are the threads that are not bound to specific CPUs. If a kernel thread
is bound to a specific CPU, then it is generally not a good idea to move that
thread to the system set because at worst it may hang the system and at best
it will slow the system down significantly. These threads are usually the IRQ
threads on a real time Linux kernel, for example, and you may want to not move
these kernel threads into system. If you leave them in the root cpuset,
then they will have access to all CPUs.
However, if your application demands an even "quieter" shield, then you can
move all movable kernel threads into the unshielded system set with the
following command.
[zuul:cpuset-trunk]# cset shield -k on
cset: --> activating kthread shielding
cset: kthread shield activated, moving 70 tasks into system cpuset...
[==================================================]%
cset: done
You can see that this moved an additional 70 tasks to the unshielded system
cpuset. Note that the -k/--kthread on parameter can be given at the shield
creation time as well and you do not need to perform these two steps
separately if you know that you will want kernel thread shielding as well.
Executing cset shield again shows us the current state of the shield.
[zuul:cpuset-trunk]# cset shield
cset: --> shielding system active with
cset: "system" cpuset of CPUSPEC(0) with 246 tasks running
cset: "user" cpuset of CPUSPEC(1-3) with 0 tasks running
You can get a detailed listing of what is running in the shield by specifying
either -s/--shield or -u/--unshield to the shield subcommand and using
the verbose flag. You will get output similar to the following.
[zuul:cpuset-trunk]# cset shield --unshield -v
cset: "system" cpuset of CPUSPEC(0) with 251 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
root 1 0 Soth init [5]
root 2 0 Soth [kthreadd]
root 84 2 Sf50 [IRQ-9]
...
alext 31796 31789 Soth less
root 32653 25222 Roth python ./cset shield --unshield -v
Note that I abbreviated the listing; we do have 251 tasks running in the
system set. The output is self-explanatory; however, the "SPPr" field may
need a little explanation. "SPPr" stands for State, Policy and Priority. You
can see that the initial two tasks are Stopped and running in timeshare
priority, marked as "oth" (for "other"). The [IRQ-9] task is also stopped,
but marked at real time FIFO policy with a priority of 50. The last task in
the listing is the cset command itself and is marked as running. Also note
that adding a second -v/--verbose option will not restrict the output to
fit into an 80 character screen.
Tear down of the shield, stopping the shield in other words, is done with the
-r/--reset option to the shield subcommand. When this command is issued,
both the system and user cpusets are deleted and any tasks that are
running in both of those cpusets are moved to the root cpuset. Once so
moved, all tasks will have access to all resources on the system. For
example:
[zuul:cpuset-trunk]# cset shield --reset
cset: --> deactivating/reseting shielding
cset: moving 0 tasks from "/user" user set to root set...
cset: moving 250 tasks from "/system" system set to root set...
[==================================================]%
cset: deleting "/user" and "/system" sets
cset: done
2.1.2. Moving Interesting Tasks Into and Out of the Shield
Now that we have a shield running, the objective is to run our "important"
processes in that shield. These processes can be anything, but usually they
are directly related to the purpose of the machine. There are two ways to run
tasks in the shield:
-
Exec a process into the shield
-
Move an already running task into the shield
Execing a Process into the Shield
Running a new process in the shield can be done with the -e/--exec option
to the shield subcommand. This is the simplest way to get a task to run in
the shield. For this example, let’s exec a new bash shell into the shield
with the following commands.
[zuul:cpuset-trunk]# cset shield -s
cset: "user" cpuset of CPUSPEC(1-3) with 0 tasks running
cset: done
[zuul:cpuset-trunk]# cset shield -e bash
cset: --> last message, executed args into cpuset "/user", new pid is: 13300
[zuul:cpuset-trunk]# cset shield -s -v
cset: "user" cpuset of CPUSPEC(1-3) with 2 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
root 13300 8583 Soth bash
root 13329 13300 Roth python ./cset shield -s -v
[zuul:cpuset-trunk]# exit
[zuul:cpuset-trunk]# cset shield -s
cset: "user" cpuset of CPUSPEC(1-3) with 0 tasks running
cset: done
The first command above lists the status of the shield. We see that the
shield is defined as CPUs 1 through 3 inclusive and currently there are no
tasks running in it.
The second command execs the bash shell into the shield with the -e
option. The last message of cset lists the PID of the new process.
Note
|
cset follows the tradition of separating the tool options from the
command to be execed options with a double dash (--). This is not shown in
this simple example, but if the command you want to exec also takes options,
separate them with the double dash like so: # cset shield -e mycommand -- -v
The -v will be passed to mycommand, and not to cset. |
The next command lists the status of the shield again. You will note that
there are actually two tasks running shielded: our new shell and the cset
status command itself. Remember that the cpuset property of a task is
inherited by its children. Since we ran the new shell in the shield, its
child, which is the status command, also ran in the shield.
Tip
|
Execing a shell into the shield is a useful way to experiment with
running tasks in the shield since all children of the shell will also run in
the shield. |
The last command exits the shell after which we request a shield status again
and see that once again, it does not contain any tasks.
You may have noticed in the output above that both the new shell and the
status command are running as the root user. This is because cset needs to
run as root and so all it’s children will also run as root. If you need to
run a process under a different user and or group, you may use the --user
and --group options for exec as follows.
[zuul:cpuset-trunk]# cset shield --user=alext --group=users -e bash
cset: --> last message, executed args into cpuset "/user", new pid is: 14212
alext@zuul> cset shield -s -v
cset: "user" cpuset of CPUSPEC(1-3) with 2 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
alext 14212 8583 Soth bash
alext 14241 14212 Roth python ./cset shield -s -v
Moving a Running Task into and out of the Shield
While execing a process into the shield is undoubtably useful, most of the
time, you’ll want to move already running tasks into and out of the shield.
The cset shield subcommand includes two options for doing this:
-s/--shield and -u/--unshield. These options require what’s called a
PIDSPEC (process specification) to also be specified with the -p/--pid
option. The PIDSPEC defines which tasks get operated on. The PIDSPEC can be
a single process ID, a list of process IDs separated by commas, and a list of
process ID ranges separated by dashes, groups of which are separated by
commas. For example:
-
--shield --pid 1234
-
This PIDSPEC argument specifies that PID 1234 be shielded.
-
--shield --pid 1234,42,1934,15000,15001,15002
-
This PIDSPEC argument specifies that this list of PIDs only be moved into the
shield.
-
--unshield -p 5000,5100,6010-7000,9232
-
This PIDSPEC argument specifies that PIDs 5000,5100 and 9232 be unshielded
(moved out of the shield) along with any existing PID that is in the range
6010 through 7000 inclusive.
Note
|
A range in a PIDSPEC does not have to have tasks running for every
number in that range. In fact, it is not even an error if there are no tasks
running in that range; none will be moved in that case. The range simply
specifies to act on any tasks that have a PID or TID that is within that
range. |
Use of the appropriate PIDSPEC can thus be handy to move tasks and groups of
tasks into and out of the shield. Additionally, there is one more option that
can help with multi-threaded processes, and that is the --threads flag. If
this flag is present in a shield or unshield command with a PIDSPEC and if
any of the task IDs in the PIDSPEC belong to a thread in a process container,
then all the sibling threads in that process container will get shielded or
unshielded as well. This flag provides an easy mechanism to shield/unshield
all threads of a process by simply specifying one thread in that process.
In the following example, we move the current shell into the shield with a
range PIDSPEC and back out with the bash variable for the current PID.
[zuul:cpuset-trunk]# echo $$
22018
[zuul:cpuset-trunk]# cset shield -s -p 22010-22020
cset: --> shielding following pidspec: 22010-22020
cset: done
[zuul:cpuset-trunk]# cset shield -s -v
cset: "user" cpuset of CPUSPEC(1-3) with 2 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
root 3770 22018 Roth python ./cset shield -s -v
root 22018 5034 Soth bash
cset: done
[zuul:cpuset-trunk]# cset shield -u -p $$
cset: --> unshielding following pidspec: 22018
cset: done
[zuul:cpuset-trunk]# cset shield -s
cset: "user" cpuset of CPUSPEC(1-3) with 0 tasks running
cset: done
Note
|
Ordinarily, the shield option will shield a PIDSPEC only if it is
currently running in the system set—the unshielded set. The unshield
option will unshield a PIDSPEC only if it is currently running in the user
set—the shielded set. If you want to shield/unshield a process that
happens to be running in the root set (not common), then use the --force
option for these commands. |
While basic shielding as described above is useful and a common use model for
cset, there comes a time when more functionality will be desired to
implement your strategy. To implement this, cset provides two subcommands:
set, which allows you to manipulate cpusets; and proc, which allows you to
manipulate processes within those cpusets.
3.1. The Set Subcommand
In order to do anything with cpusets, you must be able to create, adjust,
rename, move and destroy them. The set subcommand allows the management of
cpusets in such a manner.
3.1.1. Creating and Destroying Cpusets with Set
The basic syntax of set for cpuset creation is:
[zuul:cpuset-trunk]# cset set -c 1-3 -s my_cpuset1
cset: --> created cpuset "my_cpuset1"
This creates a cpuset named "my_cpuset1" with a CPUSPEC of CPU1, CPU2 and
CPU3. The CPUSPEC is the same concept as described in the "Setup and
Teardown of the Shield" section above. The set subcommand also takes a
-m/--mem option that lets you specify the memory nodes the set will use as
well as flags to make the CPUs and MEMs exclusive to the cpuset. If you are
on a non-NUMA machine, just leave the -m option out and the default memory
node 0 will be used.
Just like with shield, you can adjust the CPUs and MEMs with subsequent
calls to set. If, for example, you wish to adjust the "my_cpuset1" cpuset
to only use CPUs 1 and 3 (and omit CPU2), then issue the following command.
[zuul:cpuset-trunk]# cset set -c 1,3 -s my_cpuset1
cset: --> modified cpuset "my_cpuset
cset will then adjust the CPUs that are assigned to the "my_cpuset1" set to
only use CPU1 and CPU3.
To rename a cpuset, use the -n/--newname option. For example:
[zuul:cpuset-trunk]# cset set -s my_cpuset1 -n super_set
cset: --> renaming "/cpusets/my_cpuset1" to "super_set"
Renames the cpuset called "my_cpuset1" to "super_set".
To destroy a cpuset, use the -d/--destroy option as follows.
[zuul:cpuset-trunk]# cset set -d super_set
cset: --> processing cpuset "super_set", moving 0 tasks to parent "/"...
cset: --> deleting cpuset "/super_set"
cset: done
This command destroys the newly created cpuset called "super_set". When a
cpuset is destroyed, all the tasks running in it are moved to the parent
cpuset. The root cpuset, which always exists and always contains all CPUs,
can not be destroyed. You may also give the --destroy option a list of
cpusets to destroy.
Note
|
The cset subcommand creates the cpusets based on a mounted cpuset
filesystem. You do not need to know where that filesystem is mounted,
although it is easy to figure out (by default it’s on /cpusets). When you
give the set subcommand a name for a new cpuset, it is created wherever the
cpuset filesystem is mounted at. |
If you want to create a cpuset hierarchy, then you must give a path to the
cset set subcommand. This path will always begin with the root cpuset,
for which the path is /. For example.
[zuul:cpuset-trunk]# cset set -c 1,3 -s top_set
cset: --> created cpuset "top_set"
[zuul:cpuset-trunk]# cset set -c 3 -s /top_set/sub_set
cset: --> created cpuset "/top_set/sub_set"
These commands created two cpusets: top_set and sub_set. The top_set
uses CPU1 and CPU3. It has a subset of sub_set which only uses CPU3. Once
you have created a subset with a path, then if the name is unique, you do not
have to specify the path in order to affect it. If the name is not unique,
then cset will complain and ask you to use the path. For example:
[zuul:cpuset-trunk]# cset set -c 1,3 -s sub_set
cset: --> modified cpuset "sub_set
This command adds CPU1 to the sub_set cpuset for it’s use. Note that using
the path in this case is optional.
If you attempt to destroy a cpuset which has sub-cpusets, cset will complain
and not do it unless you use the -r/--recurse and the --force options.
If you do use --force, then all the tasks running in all subsets of the
deletion target cpuset will be moved to the target’s parent cpuset and all
cpusets.
Moving a cpuset from under a certain cpuset to a different location is
currently not implemented and is slated for a later release of cset.
3.1.2. Listing Cpusets with Set
To list cpusets, use the set subcommand with the -l/--list option. For
example:
[zuul:cpuset-trunk]# cset set -l
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
root 0-3 y 0 y 320 1 /
one 3 n 0 n 0 1 /one
This shows that there is currently one cpuset present called one. (Of course
that there is also the root set, which is always present.) The output shows
that the one cpuset has no tasks running in it. The root cpuset has 320
tasks running. The "-X" for "CPUs" and "MEMs" fields denotes whether the CPUs
and MEMs in the cpusets are marked exclusive to those cpusets. Note that the
one cpuset has subsets as indicated by a 1 in the Subs field. You can
specify a cpuset to list with the set subcommand as follows.
[zuul:cpuset-trunk]# cset set -l -s one
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
one 3 n 0 n 0 1 /one
two 3 n 0 n 0 1 /one/two
This output shows that there is a cpuset called two in cpuset one and it
also has subset. You can also ask for a recursive listing as follows.
[zuul:cpuset-trunk]# cset set -l -r
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
root 0-3 y 0 y 320 1 /
one 3 n 0 n 0 1 /one
two 3 n 0 n 0 1 /one/two
three 3 n 0 n 0 0 /one/two/three
This command lists all cpusets existing on the system since it asks for a
recursive listing beginning at the root cpuset. Incidentally, should you
need to specify the root cpuset you can use either root or / to specify it
explicitely—just remember that the root cpuset cannot be deleted or modified.
3.2. The Proc Subcommand
Now that we know how to create, rename and destroy cpusets with the set
subcommand, the next step is to manage threads and processes in those
cpusets. The subcommand to do this is called proc and it allows you to exec
processes into a cpuset, move existing tasks around existing cpusets, and list
tasks running in specified cpusets. For the following examples, let us
assume a cpuset setup of two sets as follows:
[zuul:cpuset-trunk]# cset set -l
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
root 0-3 y 0 y 309 2 /
two 2 n 0 n 3 0 /two
three 3 n 0 n 10 0 /three
3.2.1. Listing Tasks with Proc
Operation of the proc subcommand follows the same model as the set
subcommand. For example, to list tasks in a cpuset, you need to use the
-l/--list option and specify the cpuset by name or, if the name exists
multiple times in the cpuset hierarchy, by path. For example:
[zuul:cpuset-trunk]# cset proc -l -s two
cset: "two" cpuset of CPUSPEC(2) with 3 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
root 16141 4300 Soth bash
root 16171 16141 Soth bash
root 16703 16171 Roth python ./cset proc -l two
This output shows us that the cpuset called two has CPU2 only attached to it
and is running three tasks: two shells and the python command to list it.
Note that cpusets are inherited so that if a process is contained in a cpuset,
then any children it spawns also run within that set. In this case, the
python command to list set two was run from a shell already running in set
two. This can be seen by the PPID (parent process ID) of the python
command matching the PID of the shell.
Additionally, the "SPPr" field needs explanation. "SPPr" stands for State,
Policy and Priority. You can see that the initial two tasks are Stopped and
running in timeshare priority, marked as "oth" (for "other"). The last task
is marked as running, "R" and also at timeshare priority, "oth." If any of
these tasks would have been at real time priority, then the policy would be
shown as "f" for FIFO or "r" for round robin, and the priority would be a
number from 1 to 99. See below for an example.
[zuul:cpuset-trunk]# cset proc -l -s root | head -7
cset: "root" cpuset of CPUSPEC(0-3) with 309 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
root 1 0 Soth init [5]
root 2 0 Soth [kthreadd]
root 3 2 Sf99 [migration/0]
root 4 2 Sf99 [posix_cpu_timer]
This output shows the first few tasks in the root cpuset. Note that both
init and [kthread] are running at timeshare; however, the [migration/0]
and [posix_cpu_timer] kernel threads are running at real time policy of FIFO
and priority of 99. Incidentally, this output is from a system running the
real time Linux kernel which runs some kernel threads at real time
priorities. And finally, note that you can of course use cset as any other
Linux tool and include it in pipelines as in the example above.
Taking a peek into the third cpuset called three, we see:
[zuul:cpuset-trunk]# cset proc -l -s three
cset: "three" cpuset of CPUSPEC(3) with 10 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
alext 16165 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16169 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16170 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16237 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16491 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16492 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16493 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 17243 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 17244 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 17265 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
This output shows that a lot of beagled tasks are running in this cpuset and
it also shows an ellipsis (…) at the end of their listings. If you see this
ellipsis, that means that the command was too long to fit onto an 80 character
screen. To see the entire commandline, use the -v/--verbose flag, as per
following.
[zuul:cpuset-trunk]# cset proc -l -s three -v | head -4
cset: "three" cpuset of CPUSPEC(3) with 10 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
alext 16165 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg --autostarted --indexing-delay 300
3.2.2. Execing Tasks with Proc
To exec a task into a cpuset, the proc subcommand needs to be employed with
the -e/--exec option. Let’s exec a shell into the cpuset named two in
our set. First we check to see what is running that set:
[zuul:cpuset-trunk]# cset proc -l -s two
cset: "two" cpuset of CPUSPEC(2) with 0 tasks running
[zuul:cpuset-trunk]# cset proc -s two -e bash
cset: --> last message, executed args into cpuset "/two", new pid is: 20955
[zuul:cpuset-trunk]# cset proc -l -s two
cset: "two" cpuset of CPUSPEC(2) with 2 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
root 20955 19253 Soth bash
root 20981 20955 Roth python ./cset proc -l two
You can see that initially, two had nothing running in it. After the
completion of the second command, we list two again and see that there are
two tasks running: the shell which we execed and the python cset command
that is listing the cpuset. The reason for the second task is that the cpuset
property of a running task is inherited by all its children. Since we
executed the listing command from the new shell which was bound to cpuset
two, the resulting process for the listing is also bound to cpuset two.
Let’s test that by just running a new shell with no prefixed cset command.
[zuul:cpuset-trunk]# bash
[zuul:cpuset-trunk]# cset proc -l -s two
cset: "two" cpuset of CPUSPEC(2) with 3 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
root 20955 19253 Soth bash
root 21118 20955 Soth bash
root 21147 21118 Roth python ./cset proc -l two
Here again we see that the second shell, PID 21118, has a parent PID of 20955
which is the first shell. Both shells as well as the listing command are
running in the two cpuset.
Note
|
cset follows the tradition of separating the tool options from the
command to be execed options with a double dash (--). This is not shown in
this simple example, but if the command you want to exec also takes options,
separate them with the double dash like so: # cset proc -s myset -e mycommand
-- -v The -v will be passed to mycommand, and not to cset. |
Tip
|
Execing a shell into a cpuset is a useful way to experiment with running
tasks in that cpuset since all children of the shell will also run in the same
cpuset. |
Finally, if you misspell the command to be execed, the result may be
puzzling. For example:
[zuul:cpuset-trunk]# cset proc -s two -e blah-blah
cset: --> last message, executed args into cpuset "/two", new pid is: 21655
cset: **> [Errno 2] No such file or directory
The result is no new process even though a new PID is output. The reason for
the message is of course that the cset process forked in preparation for
exec, but the command blah-blah was not found in order to exec it.
3.2.3. Moving Tasks with Proc
Although the ability to exec a task into a cpuset is fundamental, you will
most likely be moving tasks between cpusets more often. Moving tasks is
accomplished with the -m/--move and -p/--pid options to the proc
subcommand of cset. The move option tells the proc subcommand that a task
move is requested. The -p/--pid option takes an argument called a PIDSPEC
(PID Specification). The PIDSPEC defines which tasks get operated on.
The PIDSPEC can be a single process ID, a list of process IDs separated by
commas, and a list of process ID ranges also separated by commas. For
example:
-
--move --pid 1234
-
This PIDSPEC argument specifies that task 1234 be moved.
-
--move --pid 1234,42,1934,15000,15001,15002
-
This PIDSPEC argument specifies that this list of tasks only be moved.
-
--move --pid 5000,5100,6010-7000,9232
-
This PIDSPEC argument specifies that tasks 5000,5100 and 9232 be moved along
with any existing task that is in the range 6010 through 7000 inclusive.
Note
|
A range in a PIDSPEC does not have to have running tasks for every
number in that range. In fact, it is not even an error if there are no tasks
running in that range; none will be moved in that case. The range simply
specifies to act on any tasks that have a PID or TID that is within that
range. |
In the following example, we move the current shell into the cpuset named
two with a range PIDSPEC and back out to the root cpuset with the bash
variable for the current PID.
[zuul:cpuset-trunk]# cset proc -l -s two
cset: "two" cpuset of CPUSPEC(2) with 0 tasks running
[zuul:cpuset-trunk]# echo $$
19253
[zuul:cpuset-trunk]# cset proc -m -p 19250-19260 -t two
cset: moving following pidspec: 19253
cset: moving 1 userspace tasks to /two
cset: done
[zuul:cpuset-trunk]# cset proc -l -s two
cset: "two" cpuset of CPUSPEC(2) with 2 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
root 19253 16447 Roth bash
root 29456 19253 Roth python ./cset proc -l -s two
[zuul:cpuset-trunk]# cset proc -m -p $$ -t root
cset: moving following pidspec: 19253
cset: moving 1 userspace tasks to /
cset: done
[zuul:cpuset-trunk]# cset proc -l -s two
cset: "two" cpuset of CPUSPEC(2) with 0 tasks running
Use of the appropriate PIDSPEC can thus be handy to move tasks and groups of
tasks. Additionally, there is one more option that can help with
multi-threaded processes, and that is the --threads flag. If this flag is
present in a proc move command with a PIDSPEC and if any of the task IDs in
the PIDSPEC belongs to a thread in a process container, then all the sibling
threads in that process container will also get moved. This flag provides an
easy mechanism to move all threads of a process by simply specifying one
thread in that process. In the following example, we move all the threads
running in cpuset three to cpuset two by using the --threads flag.
[zuul:cpuset-trunk]# cset set two three
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
two 2 n 0 n 0 0 /two
three 3 n 0 n 10 0 /three
[zuul:cpuset-trunk]# cset proc -l -s three
cset: "three" cpuset of CPUSPEC(3) with 10 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
alext 16165 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16169 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16170 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16237 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16491 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16492 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16493 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 17243 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 17244 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 27133 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
[zuul:cpuset-trunk]# cset proc -m -p 16165 --threads -t two
cset: moving following pidspec: 16491,16493,16492,16170,16165,16169,27133,17244,17243,16237
cset: moving 10 userspace tasks to /two
[==================================================]%
cset: done
[zuul:cpuset-trunk]# cset set two three
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
two 2 n 0 n 10 0 /two
three 3 n 0 n 0 0 /three
Moving All Tasks from one Cpuset to Another
There is a special case for moving all tasks currently running in one cpuset
to another. This can be a common use case, and when you need to do it,
specifying a PIDSPEC with -p is not necessary so long as you use the
-f/--fromset and the -t/--toset options.
In the following example, we move all 10 beagled threads back to cpuset
three with this method.
[zuul:cpuset-trunk]# cset proc -l two three
cset: "two" cpuset of CPUSPEC(2) with 10 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
alext 16165 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16169 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16170 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16237 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16491 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16492 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 16493 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 17243 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 17244 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
alext 27133 1 Soth beagled /usr/lib64/beagle/BeagleDaemon.exe --bg -...
cset: "three" cpuset of CPUSPEC(3) with 0 tasks running
[zuul:cpuset-trunk]# cset proc -m -f two -t three
cset: moving all tasks from two to /three
cset: moving 10 userspace tasks to /three
[==================================================]%
cset: done
[zuul:cpuset-trunk]# cset set two three
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
two 2 n 0 n 0 0 /two
three 3 n 0 n 10 0 /three
Moving Kernel Threads with Proc
Kernel threads are special and cset detects tasks that are kernel threads
and will refuse to move them unless you also add a -k/--kthread option to
your proc move command. Even if you include -k, cset will still
refuse to move the kernel thread if they are bound to specific CPUs. The
reason for this is system protection.
A number of kernel threads, especially on the real time Linux kernel, are
bound to specific CPUs and depend on per-CPU kernel variables. If you move
these threads to a different CPU than what they are bound to, you risk at best
that the system will become horribly slow, and at worst a system hang. If you
still insist to move those threads (after all, cset needs to give the
knowledgeable user access to the keys), then you need to use the --force
option additionally.
Warning
|
Overriding a task move command with --force can have dire
consequences for the system. Please be sure of the command before you force
it. |
In the following example, we move all unbound kernel threads running in the
root cpuset to the cpuset named two by using the -k option.
[zuul:cpuset-trunk]# cset proc -k -f root -t two
cset: moving all kernel threads from / to /two
cset: moving 70 kernel threads to: /two
cset: --> not moving 76 threads (not unbound, use --force)
[==================================================]%
cset: done
You will note that we used the fromset→toset facility of the proc
subcommand and we only specified the -k option (not the -m option). This
has the effect of moving all kernel threads only.
Note that only 70 actual kernel threads were moved and 76 were not. The
reason that 76 kernel threads were not moved was because they are bound to
specific CPUs. Now, let’s move those kernel threads back to root.
[zuul:cpuset-trunk]# cset proc -k -f two -t root
cset: moving all kernel threads from /two to /
cset: ** no task matched move criteria
cset: **> kernel tasks are bound, use --force if ok
[zuul:cpuset-trunk]# cset set -l -s two
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
two 2 n 0 n 70 0 /two
Ah! What’s this? Cset refused to move the kernel threads back to root
because it says that they are "bound." Let’s check this with the Linux
taskset command.
[zuul:cpuset-trunk]# cset proc -l -s two | head -5
cset: "two" cpuset of CPUSPEC(2) with 70 tasks running
USER PID PPID SPPr TASK NAME
-------- ----- ----- ---- ---------
root 2 0 Soth [kthreadd]
root 55 2 Soth [khelper]
[zuul:cpuset-trunk]# taskset -p 2
pid 2's current affinity mask: 4
[zuul:cpuset-trunk]# cset set -l -s two
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
two 2 n 0 n 70 0 /two
Of course, since the cpuset named two only has CPU2 assigned to it, once we
moved the unbound kernel threads from root to two, their affinity masks
got automatically changed to only use CPU2. This is evident from the
taskset output which is a hex value. To really move these threads back to
root, we need to force the move as follows.
[zuul:cpuset-trunk]# cset proc -k -f two -t root --force
cset: moving all kernel threads from /two to /
cset: moving 70 kernel threads to: /
[==================================================]%
cset: done
3.2.4. Destroying Tasks
There actually is no cset subcommand or option to destroy tasks—it’s not
really needed. Tasks exist and are accessible on the system as normal, even
if they happen to be running in one cpuset or another. To destroy tasks, use
the usual ^C method or by using the kill(1) command.
3.3. Implementing "Shielding" with Set and Proc
With the preceding material on the set and proc subcommands, we now have
the background to implement the basic shielding model, just like the shield
subcommand.
One may pose the question as to why we want to do this, especially since
shield already does it? The answer is that sometimes one needs more
functionality than shield has to implement one’s shielding strategy. In
those case you need to first stop using shield since that subcommand will
interfere with the further application of set and proc; however, you will
still need to implement the functionality of shield in order to implement
successful shielding.
Remember from the above sections describing shield, that shielding has at
minimum three cpusets: root, which is always present and contains all CPUs;
system which is the "non-shielded" set of CPUs and runs unimportant system
tasks; and user, which is the "shielded" set of CPUs and runs your important
tasks. Remember also that shield moves all movable tasks into system
and, optionally, moves unbound kernel threads into system as well.
We start first by creating the system and user cpusets as follows. We
assume that the machine is a four-CPU machine without NUMA memory features.
The system cpuset should hold only CPU0 while the user cpuset should hold
the rest of the CPUs.
[zuul:cpuset-trunk]# cset set -c 0 -s system
cset: --> created cpuset "system"
[zuul:cpuset-trunk]# cset set -c 1-3 -s user
cset: --> created cpuset "user"
[zuul:cpuset-trunk]# cset set -l
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
root 0-3 y 0 y 333 2 /
user 1-3 n 0 n 0 0 /user
system 0 n 0 n 0 0 /system
Now, we need to move all running user processes into the system cpuset.
[zuul:cpuset-trunk]# cset proc -m -f root -t system
cset: moving all tasks from root to /system
cset: moving 188 userspace tasks to /system
[==================================================]%
cset: done
[zuul:cpuset-trunk]# cset set -l
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
root 0-3 y 0 y 146 2 /
user 1-3 n 0 n 0 0 /user
system 0 n 0 n 187 0 /system
We now have the basic shielding set up. Since all userspace tasks are running
in system, anything that is spawned from them will also run in system.
The user cpuset has nothing running in it unless you put tasks there with
the proc subcommand as described above. If you also want to move movable
kernel threads from root to system (in order to achieve a form of
"interrupt shielding" on a real time Linux kernel for example), you would
execute the following command as well.
[zuul:cpuset-trunk]# cset proc -k -f root -t system
cset: moving all kernel threads from / to /system
cset: moving 70 kernel threads to: /system
cset: --> not moving 76 threads (not unbound, use --force)
[==================================================]%
cset: done
[zuul:cpuset-trunk]# cset set -l
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
root 0-3 y 0 y 76 2 /
user 1-3 n 0 n 0 0 /user
system 0 n 0 n 257 0 /system
At this point, you have achieved the simple shielding model that the shield
subcommand provides. You can now add other cpuset definitions to expand your
shielding strategy beyond that simple model.
3.4. Implementing Hierarchy with Set and Proc
One popular extended "shielding" model is based on hierarchical cpusets, each
with diminishing numbers of CPUs. This model is used to create "priority
cpusets" that allow assignment of CPU resources to tasks based on some
arbitrary priority definition. The idea being that a higher priority task
will get access to more CPU resources than a lower priority task.
The example provided here once again assumes a machine with four CPUs and no
NUMA memory features. This base serves to illustrate the point well; however,
note that if your machine has (many) more CPUs, then strategies such as this
and others get more interesting.
We define a shielding set up as in the previous section where we have a
system cpuset with just CPU0 that takes care of "unimportant" system tasks.
One usually requires this type of cpuset since it forms the basis of
shielding. We modify the strategy to not use a user cpuset, instead we
create a number of new cpusets each holding one more CPU than the other.
These cpusets will be called prio_low with one CPU, prio_med with two
CPUs, prio_high with three CPUs, and prio_all with all CPUs.
Note
|
One may ask, why create a prio_all with all CPUs when that is
substantially the definition of the root cpuset? The answer is that it is
best to keep a separation between the root cpuset and everything else, even
if a particular cpuset duplicates root exactly. Usually, one builds
automation on top of a cpuset strategy. In these cases, it is best to avoid
using invariant names of cpusets, such as root for example, in this
automation. |
All of these prio_* cpusets can be created under root, in a flat way;
however, it is advantageous to create them as a hierarchy. The reasoning for
this is twofold: first, if a cpuset is destroyed, all its tasks are moved to
its parent; second, one can use exclusive CPUs in a hierarchy.
There is a planned addition to the proc subcommand that will allow moving a
specified PIDSPEC of tasks running in a specified cpuset to its parent. This
addition will ease the automation burden.
If a cpuset has CPUs that are exclusive to it, then other cpusets may not make
use of those CPUs unless they are children of that cpuset. This has more
relevance to machines with many CPUs and more complex strategies.
Now, we start with a clean slate and build the appropriate cpusets as
follows.
[zuul:cpuset-trunk]# cset set -r
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
root 0-3 y 0 y 344 0 /
[zuul:cpuset-trunk]# cset set -c 0-3 prio_all
cset: --> created cpuset "prio_all"
[zuul:cpuset-trunk]# cset set -c 1-3 /prio_all/prio_high
cset: --> created cpuset "/prio_all/prio_high"
[zuul:cpuset-trunk]# cset set -c 2-3 /prio_all/prio_high/prio_med
cset: --> created cpuset "/prio_all/prio_high/prio_med"
[zuul:cpuset-trunk]# cset set -c 3 /prio_all/prio_high/prio_med/prio_low
cset: --> created cpuset "/prio_all/prio_high/prio_med/prio_low"
[zuul:cpuset-trunk]# cset set -c 0 system
cset: --> created cpuset "system"
[zuul:cpuset-trunk]# cset set -l -r
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
root 0-3 y 0 y 344 2 /
system 0 n 0 n 0 0 /system
prio_all 0-3 n 0 n 0 1 /prio_all
prio_high 1-3 n 0 n 0 1 /prio_all/prio_high
prio_med 2-3 n 0 n 0 1 /prio_all/prio_high/prio_med
prio_low 3 n 0 n 0 0 /prio_all/pr...rio_med/prio_low
Note
|
We used the -r/--recurse switch to list all the sets in the last
command above. If we had not, then the prio_med and prio_low cpusets
would not have been listed. |
The strategy is now implemented and we now move all userspace tasks as well as
all movable kernel threads into the system cpuset to activate the shield.
[zuul:cpuset-trunk]# cset proc -m -k -f root -t system
cset: moving all tasks from root to /system
cset: moving 198 userspace tasks to /system
cset: moving 70 kernel threads to: /system
cset: --> not moving 76 threads (not unbound, use --force)
[==================================================]%
cset: done
[zuul:cpuset-trunk]# cset set -l -r
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
root 0-3 y 0 y 76 2 /
system 0 n 0 n 268 0 /system
prio_all 0-3 n 0 n 0 1 /prio_all
prio_high 1-3 n 0 n 0 1 /prio_all/prio_high
prio_med 2-3 n 0 n 0 1 /prio_all/prio_high/prio_med
prio_low 3 n 0 n 0 0 /prio_all/pr...rio_med/prio_low
The shield is now active. Since the prio_* cpuset names are unique, one can
assign tasks to them either via either their simple name, or their full path
(as described in the proc section above).
You may have noted that there is an ellipsis in the path of the prio_low
cpuset in the listing above. This is done in order to fit the output onto an
80 character screen. If you want to see the entire line, then you need to use
the -v/--verbose flag as follows.
[zuul:cpuset-trunk]# cset set -l -r -v
cset:
Name CPUs-X MEMs-X Tasks Subs Path
------------ ---------- - ------- - ----- ---- ----------
root 0-3 y 0 y 76 2 /
system 0 n 0 n 268 0 /system
prio_all 0-3 n 0 n 0 1 /prio_all
prio_high 1-3 n 0 n 0 1 /prio_all/prio_high
prio_med 2-3 n 0 n 0 1 /prio_all/prio_high/prio_med
prio_low 3 n 0 n 0 0 /prio_all/prio_high/prio_med/prio_low
The commands listed in the previous sections always used all the required
options. Cset however does have a shortcut facility that will execute
certain commands without specifying all options. An effort has been made to
do this with the "principle of least surprise." This means that if you do not
specify options, but do specify parameters, then the outcome of the command
should be intuitive. As much as possible.
Using shortcuts is of course not necessary. In fact, you can not only not use
shortcuts, but you can use long options instead of short, in case you really
enjoy typing… All kidding aside, using long options and not using shortcuts
does have a use case: when you write a script intended to be self-documenting,
or perhaps when you generate cset documentation.
To begin, the subcommands shield, set and proc can themselves be
shortened to the fewest number of characters that are unambiguous. For
example, the following commands are identical:
# cset shield -s -p 1234 <--> # cset sh -s -p 1234
# cset set -c 1,3 -s newset <--> # cset se -c 1,3 -s newset
# cset proc -s newset -e bash <--> # cset p -s newset -e bash
Note that proc can be shortened to just p, while shield and set need
two letters to disambiguate.
4.1. Shield Subcommand Shortcuts
The shield subcommand supports two areas with shortcuts: the case when there
is no options given where to shield is the common use case, and making the
-p/--pid option optional for the -s/--shield and -u/--unshield
options.
For the common use case of actually shielding either a PIDSPEC or execing a
command into the shield, the following cset commands are equivalent.
# cset shield -s -p 1234,500-649 <--> # cset sh 1234,500-649
# cset shield -s -e bash <--> # cset sh bash
When using the -s or -u shield/unshield options, it is optional to use the
-p option to specify a PIDSPEC. For example:
# cset shield -s -p 1234 <--> # cset sh -s 1234
# cset shield -u -p 1234 <--> # cset sh -u 1234
4.2. Set Subcommand Shortcuts
The set subcommand has a limited number of shortcuts. Basically, the
-s/--set option is optional in most cases and the -l/--list option is
also optional if you want to list sets. For example, these commands are
equivalent.
# cset set -l -s myset <--> # cset se -l myset
# cset se -l myset <--> # cset se myset
# cset set -c 1,2,3 -s newset <--> # cset se -c 1,2,3 newset
# cset set -d -s newset <--> # cset se -d newset
# cset set -n newname -s oldname <--> # cset se -n newname oldname
In fact, if you want to apply either the list or the destroy options to
multiple cpusets with one cset command, you’ll need to not use the -s
option. For example:
# cset se -d myset yourset ourset
--> destroys cpusets: myset, yourset and ourset
# cset se -l prio_high prio_med prio_low
--> lists only cpusets prio_high, prio_med and prio_low
--> the -l is optional in this case since list is default
4.3. Proc Subcommand Shortcuts
For the proc subcommand, the -s, -t and -f options to specify the
cpuset, the origination cpuset and the destination cpuset, can sometimes be
optional. For example, the following commands are equivalent.
To list tasks in cpusets:
# cset proc -l -s myset \
# cset proc -l -f myset --> # cset p -l myset
# cset proc -l -t myset /
# cset p -l myset <--> # cset p myset
# cset proc -l -s one two <--> # cset p -l one two
# cset p -l one two <--> # cset p one two
To exec a process into a cpuset:
# cset proc -s myset -e bash <--> # cset p myset -e bash
Movement of tasks into and out of cpusets have the following shortcuts.
To move a PIDSPEC into a cpuset:
# cset proc -m -p 4242,4243 -s myset <--> # cset p -m 4242,4243 myset
# cset proc -m -p 12 -t myset <--> # cset p -m 12 myset
To move all tasks from one cpuset to another:
# cset proc -m -f set1 -t set2 \
# cset proc -m -s set1 -t set2 --> # cset p -m set1 set2
# cset proc -m -f set1 -s set2 /