linux-kernel - Re: [PATCH 3/6] psi: eliminate lazy clock mode

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Message-ID: <CAJuCfpFtqd=RDDW_U5HFXuAZzNh6F+Enrjz3P4jZg=hvyH9RwQ@mail.gmail.com>
Date:   Mon, 17 Dec 2018 17:10:01 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>, lizefan@...wei.com,
        Johannes Weiner <hannes@...xchg.org>, axboe@...nel.dk,
        dennis@...nel.org, Dennis Zhou <dennisszhou@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, cgroups@...r.kernel.org,
        linux-mm <linux-mm@...ck.org>, linux-doc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@...roid.com
Subject: Re: [PATCH 3/6] psi: eliminate lazy clock mode

On Mon, Dec 17, 2018 at 6:58 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Dec 14, 2018 at 09:15:05AM -0800, Suren Baghdasaryan wrote:
> > Eliminate the idle mode and keep the worker doing 2s update intervals
> > at all times.
>
> That sounds like a bad deal.. esp. so for battery powered devices like
> say Andoird.
>
> In general the push has been to always idle everything, see NOHZ and
> NOHZ_FULL and all the work that's being put into getting rid of any and
> all period work.

Thanks for the feedback Peter! The removal of idle mode is unfortunate
but so far we could not find an elegant solution to handle 3 states
(IDLE / REGULAR / POLLING) without additional synchronization inside
the hotpath. The issue, as I remember it, was that while scheduling a
regular update inside psi_group_change() (IDLE to REGULAR transition)
we might override an earlier update being scheduled inside
psi_update_work(). I think we can solve that by using
mod_delayed_work_on() inside psi_update_work() but I might be missing
some other race. I'll discuss this again with Johannes and see if we
can synchronize all states using only atomic operations on clock_mode.

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