linux-kernel - Re: sched/fair: scheduler not running high priority process on idle cpu

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Message-ID: <20200115145645.GM2827@hirez.programming.kicks-ass.net>
Date:   Wed, 15 Jan 2020 15:56:45 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Steven Rostedt' <rostedt@...dmis.org>,
        'Vincent Guittot' <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: sched/fair: scheduler not running high priority process on idle
 cpu

On Wed, Jan 15, 2020 at 12:44:19PM +0000, David Laight wrote:

> Code that runs with a spin-lock held (or otherwise disables preemption)
> for significant periods probably ought to be detected and warned.
> I'm not sure of a suitable limit, 100us is probably excessive on x86.

Problem is, without CONFIG_PREEMPT_COUNT (basically only
PREEMPT/PREEMPT_RT) we can't even tell.

And I think we tried adding warnings to things like softirq, but then we
get into arguments with the pure performance people on how allowing it
longer will make their benchmarks go faster.

There really is no silver bullet here :/