Messages in this thread | | | Date | Wed, 12 Apr 2023 14:40:53 -0400 | Subject | Re: [PATCH v4 5/5] cgroup/cpuset: Optimize out unneeded cpuset_can_fork/cpuset_cancel_fork calls | From | Waiman Long <> |
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On 4/12/23 14:27, Tejun Heo wrote: > On Tue, Apr 11, 2023 at 09:36:01AM -0400, Waiman Long wrote: >> The newly introduced cpuset_can_fork() and cpuset_cancel_fork() calls >> are only needed when the CLONE_INTO_CGROUP flag is set which is not >> likely. Adding an extra cpuset_can_fork() call does introduce a bit >> of performance overhead in the fork/clone fastpath. To reduce this >> performance overhead, introduce a new clone_into_cgroup_can_fork flag >> into the cgroup_subsys structure. This flag, when set, will call the >> can_fork and cancel_fork methods only if the CLONE_INTO_CGROUP flag >> is set. >> >> The cpuset code is now modified to set this flag. The same cpuset >> checking code in cpuset_can_fork() and cpuset_cancel_fork() will have >> to stay as the cgroups can be different, but the cpusets may still be >> the same. So the same check must be present in both cpuset_fork() and >> cpuset_can_fork() to make sure that attach_in_progress is correctly set. >> >> Signed-off-by: Waiman Long <longman@redhat.com> > Waiman, I'm not necessarily against this optimization but can we at least > have some performance numbers to show that this is actually meaningful? > Given how heavy our fork path is, I'm not too sure this would show up in any > meaningful way.
That make sense to me. I am OK to leave it for now as it is an optimization patch anyway.
BTW, another question that I have is about the cgroup_threadgroup_rwsem. It is currently a percpu rwsem. Is it possible to change it into a regular rwsem instead? It is causing quite a bit of latency for workloads that require rather frequent changes to cgroups. I know we have a "favordynmods" mount option to disable the percpu operation. This will still be less performant than a normal rwsem. Of course the downside is that the fork/exit fastpaths will be slowed down a bit.
Thanks, Longman
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