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

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Date:   Wed, 3 Aug 2022 09:48:18 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Chengming Zhou <zhouchengming@...edance.com>, 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

Hello,

On Wed, Aug 03, 2022 at 03:22:16PM -0400, Johannes Weiner wrote:
> Where it gets trickier is also stopping the tracking of task counts in
> a cgroup. For re-enabling afterwards, we'd have to freeze scheduler
> and cgroup state and find all tasks of interest across all CPUs for
> the given cgroup to recreate the counts. I'm not quite sure whether
> that's feasible, and if so, whether it's worth the savings.

If this turns out to be necessary, I wonder whether we can just be
opportunistic. ie. don't bother with walking all the tasks but only remember
whether a task is accounted at a given level or not (this can be a bitmap
which is allocated at cgroup attach type and in most caess will be pretty
small). Then, maybe we can just start accounting them as they cycle through
state transitions - we ignore the ones leaving states that they weren't
accounted for and start remembering the new states they enter.

Thanks.

-- 
tejun