linux-kernel - Re: [RFC] schedutil: Fix iowait boost issues for slow I/O devices

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Message-ID: <YMxagWy/Ioj3aere@hirez.programming.kicks-ass.net>
Date:   Fri, 18 Jun 2021 10:34:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>, Josh Don <joshdon@...gle.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Mel Gorman <mgorman@...e.de>,
        Qais Yousef <qais.yousef@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>, rostedt@...dmis.org
Subject: Re: [RFC] schedutil: Fix iowait boost issues for slow I/O devices


Aside from everything else:

On Fri, Jun 18, 2021 at 12:06:39AM -0400, Joel Fernandes (Google) wrote:

> +unsigned int sysctl_iowait_reset_ticks = 20;
> +unsigned int sysctl_iowait_apply_ticks = 10;

Never expose HZ to userspace. we even have USER_HZ (no new usage please)
that fixed existing problems with that.

If you expose a knob, make it in sane time units and convert internally
to whatever you need. That said; less is more with knobs.