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

[Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On 4/12/23 15:17, Tejun Heo wrote:
> Hello,
>
> On Wed, Apr 12, 2023 at 02:40:53PM -0400, Waiman Long wrote:
>> 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-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
>>> 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.
> I don't know. Maybe? A rwsem actually has a scalability factor in that the
> more CPUs are forking, the more expensive the rwsem becomes, so it is a bit
> more of a concern. Another factor is that in majority of use cases we're
> almost completely bypassing write-locking percpu_rwsem, so it feel a bit sad
> to convert it to a regular rwsem. So, if favordynmods is good enough, I'd
> like to keep it that way.

It is just a thought that I have since Juri is in the process of 
reverting the change of cpuset_mutex to cpuset_rwsem. Percpu rwsem can 
be a bit problematic in PREEMPT_RT kernel since it does not support 
proper priority inheritance though.

Cheers,
Longman