linux-kernel - Re: [PATCH] sched: Warn on long periods of pending need

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Date:   Wed, 17 Mar 2021 09:31:41 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Josh Don <joshdon@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        David Rientjes <rientjes@...gle.com>,
        Oleg Rombakh <olegrom@...gle.com>, Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH] sched: Warn on long periods of pending need_resched


* Josh Don <joshdon@...gle.com> wrote:

> +static inline u64 resched_latency_check(struct rq *rq)
> +{
> +	int latency_warn_ms = READ_ONCE(sysctl_resched_latency_warn_ms);
> +	bool warn_only_once = (latency_warn_ms == RESCHED_DEFAULT_WARN_LATENCY_MS);
> +	u64 need_resched_latency, now = rq_clock(rq);
> +	static bool warned_once;
> +
> +	if (warn_only_once && warned_once)
> +		return 0;
> +
> +	if (!need_resched() || latency_warn_ms < 2)
> +		return 0;
> +
> +	/* Disable this warning for the first few mins after boot */
> +	if (now < RESCHED_BOOT_QUIET_SEC * NSEC_PER_SEC)
> +		return 0;
> +
> +	if (!rq->last_seen_need_resched_ns) {
> +		rq->last_seen_need_resched_ns = now;
> +		rq->ticks_without_resched = 0;
> +		return 0;
> +	}
> +
> +	rq->ticks_without_resched++;

So AFAICS this will only really do something useful on full-nohz 
kernels with sufficiently long scheduler ticks, right?

On other kernels the scheduler tick interrupt, when it returns to 
user-space, will trigger a reschedule if it sees a need_resched.

Thanks,

	Ingo