linux-kernel - Re: [PATCH] sched/core: forced idle accounting

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Date:   Thu, 14 Oct 2021 16:24:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Don <joshdon@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Vineeth Pillai <vineethrp@...il.com>,
        Hao Luo <haoluo@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/core: forced idle accounting

On Tue, Oct 12, 2021 at 12:45:28PM -0700, Josh Don wrote:
> On Tue, Oct 12, 2021 at 5:27 AM Peter Zijlstra <peterz@...radead.org> wrote:

> > > We scale by the number of cpus actually forced idle, since we don't
> > > want to falsely over or under charge forced idle time (defined
> > > strictly as time where we have a runnable task but idle the cpu). The
> > > more important scaling here though is the division over the number of
> > > running entities. This is done so that the aggregate amount of forced
> > > idle over some group of threads makes sense. Ie if we have a cpu with
> > > SMT8, and a group of 7 threads sharing a cookie, we don't want to
> > > accrue 7 units of forced idle time per unit time while the 8th SMT is
> > > forced idle.
> >
> > So why not simply compute the strict per-cpu force-idle time and let
> > userspace sort out the rest?
> 
> Do you mean to compute force idle solely as a per-cpu value? I think
> that would be fine in addition to the per-thread field, but a
> desirable property here is proper attribution to the cause of the
> force idle. That lets system management understand which jobs are the
> most antagonistic from a coresched perspective, and is a signal
> (albeit noisy, due to system state and load balancing decisions) for
> scaling their capacity requirements.

Urgh, reading is hard. I hadn't noticed you did per-task accounting (and
the original changelog doesn't clarify this either).

Also, should all this be undef SCHED_DEBUG ? Or be part of SCHEDSTATS ?