Re: [PATCH v4 5/5] cgroup/cpuset: Optimize out unneeded cpuset_can_fork/cpuset_cancel_fork calls

[Waiman Long]

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@xxxxxxxxxx>
> 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