linux-kernel - Re: [PATCH 8/9] sched/psi: add kernel cmdline parameter psi_inner

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Date:   Wed, 3 Aug 2022 20:17:22 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>
Cc:     surenb@...gle.com, mingo@...hat.com, peterz@...radead.org,
        corbet@....net, akpm@...ux-foundation.org, rdunlap@...radead.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        songmuchun@...edance.com, cgroups@...r.kernel.org
Subject: Re: [PATCH 8/9] sched/psi: add kernel cmdline parameter
 psi_inner_cgroup

On 2022/7/27 01:54, Tejun Heo wrote:
> Hello,
> 
> On Mon, Jul 25, 2022 at 12:52:17PM -0400, Johannes Weiner wrote:
>> On Thu, Jul 21, 2022 at 12:04:38PM +0800, Chengming Zhou wrote:
>>> PSI accounts stalls for each cgroup separately and aggregates it
>>> at each level of the hierarchy. This may case non-negligible overhead
>>> for some workloads when under deep level of the hierarchy.
>>>
>>> commit 3958e2d0c34e ("cgroup: make per-cgroup pressure stall tracking configurable")
>>> make PSI to skip per-cgroup stall accounting, only account system-wide
>>> to avoid this each level overhead.
>>>
>>> For our use case, we also want leaf cgroup PSI accounted for userspace
>>> adjustment on that cgroup, apart from only system-wide management.
>>
>> I hear the overhead argument. But skipping accounting in intermediate
>> levels is a bit odd and unprecedented in the cgroup interface. Once we
>> do this, it's conceivable people would like to do the same thing for
>> other stats and accounting, like for instance memory.stat.
>>
>> Tejun, what are your thoughts on this?
> 
> Given that PSI requires on-the-spot recursive accumulation unlike other
> stats, it can add quite a bit of overhead, so I'm sympathetic to the
> argument because PSI can't be made cheaper by kernel being better (or at
> least we don't know how to yet).
> 
> That said, "leaf-only" feels really hacky to me. My memory is hazy but
> there's nothing preventing any cgroup from being skipped over when updating
> PSI states, right? The state count propagation is recursive but it's each
> task's state being propagated upwards not the child cgroup's, so we can skip
> over any cgroup arbitrarily. ie. we can at least turn off PSI reporting on
> any given cgroup without worrying about affecting others. Am I correct?

Yes, I think it's correct.

> 
> Assuming the above isn't wrong, if we can figure out how we can re-enable
> it, which is more difficult as the counters need to be resynchronized with
> the current state, that'd be ideal. Then, we can just allow each cgroup to
> enable / disable PSI reporting dynamically as they see fit.

This method is more fine-grained but more difficult like you said above.
I think it may meet most needs to disable PSI stats in intermediate cgroups?

Thanks!

> 
> Thanks.
>