Re: [PATCH] sched: Warn on long periods of pending need_resched

[Ingo Molnar <mingo@kernel.org>]

* Josh Don <joshdon@google.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