linux-kernel - Re: [PATCH v4 5/5] cgroup/cpuset: Optimize out unneeded cpuset_can_fork/cpuset_cancel

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Message-ID: <90b7bc16-0673-02b7-dad1-f24bc956f1c5@redhat.com>
Date:   Wed, 12 Apr 2023 14:40:53 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Christian Brauner <brauner@...nel.org>,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Michal Koutný <mkoutny@...e.com>,
        Giuseppe Scrivano <gscrivan@...hat.com>
Subject: Re: [PATCH v4 5/5] cgroup/cpuset: Optimize out unneeded
 cpuset_can_fork/cpuset_cancel_fork calls


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@...hat.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