linux-kernel - Re: [RFC PATCH 10/10] psi: aggregate ongoing stall events when somebody reads pressure

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Date:   Fri, 13 Jul 2018 18:17:58 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Christopher Lameter <cl@...ux.com>,
        Mike Galbraith <efault@....de>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [RFC PATCH 10/10] psi: aggregate ongoing stall events when
 somebody reads pressure

On Thu, Jul 12, 2018 at 04:45:37PM -0700, Andrew Morton wrote:
> On Thu, 12 Jul 2018 13:29:42 -0400 Johannes Weiner <hannes@...xchg.org> wrote:
> 
> > Right now, psi reports pressure and stall times of already concluded
> > stall events. For most use cases this is current enough, but certain
> > highly latency-sensitive applications, like the Android OOM killer,
> > might want to know about and react to stall states before they have
> > even concluded (e.g. a prolonged reclaim cycle).
> > 
> > This patches the procfs/cgroupfs interface such that when the pressure
> > metrics are read, the current per-cpu states, if any, are taken into
> > account as well.
> > 
> > Any ongoing states are concluded, their time snapshotted, and then
> > restarted. This requires holding the rq lock to avoid corruption. It
> > could use some form of rq lock ratelimiting or avoidance.
> > 
> > Requested-by: Suren Baghdasaryan <surenb@...gle.com>
> > Not-yet-signed-off-by: Johannes Weiner <hannes@...xchg.org>
> 
> What-does-that-mean:?

I didn't think this patch was ready for upstream yet, hence the RFC
and the lack of a proper sign-off.

But Suren has been testing this and found it useful in his specific
low-latency application, so I included it for completeness, for other
testers to find, and for possible suggestions on how to improve it.