Re: [PATCH] Sched/fair: Block nohz tick_stop when cfs bandwidth in use

[Benjamin Segall <bsegall@google.com>]

Phil Auld <pauld@redhat.com> writes:

> CFS bandwidth limits and NOHZ full don't play well together.  Tasks
> can easily run well past their quotas before a remote tick does
> accounting.  This leads to long, multi-period stalls before such
> tasks can run again. Currentlyi, when presented with these conflicting
> requirements the scheduler is favoring nohz_full and letting the tick
> be stopped. However, nohz tick stopping is already best-effort, there
> are a number of conditions that can prevent it, whereas cfs runtime
> bandwidth is expected to be enforced.
>
> Make the scheduler favor bandwidth over stopping the tick by setting
> TICK_DEP_BIT_SCHED when the only running task is a cfs task with
> runtime limit enabled.
>
> Add sched_feat HZ_BW (off by default) to control this behavior.
>
> Signed-off-by: Phil Auld <pauld@redhat.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: Ben Segall <bsegall@google.com>
> ---
>  kernel/sched/fair.c     | 33 ++++++++++++++++++++++++++++++++-
>  kernel/sched/features.h |  2 ++
>  2 files changed, 34 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 373ff5f55884..880eadfac330 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6139,6 +6139,33 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
>  	rcu_read_unlock();
>  }
>  
> +#ifdef CONFIG_NO_HZ_FULL
> +/* called from pick_next_task_fair() */
> +static void sched_fair_update_stop_tick(struct rq *rq, struct task_struct *p)
> +{
> +	struct cfs_rq *cfs_rq = task_cfs_rq(p);
> +	int cpu = cpu_of(rq);
> +
> +	if (!sched_feat(HZ_BW) || !cfs_bandwidth_used())
> +		return;
> +
> +	if (!tick_nohz_full_cpu(cpu))
> +		return;
> +
> +	if (rq->nr_running != 1 || !sched_can_stop_tick(rq))
> +		return;
> +
> +	/*
> +	 *  We know there is only one task runnable and we've just picked it. The
> +	 *  normal enqueue path will have cleared TICK_DEP_BIT_SCHED if we will
> +	 *  be otherwise able to stop the tick. Just need to check if we are using
> +	 *  bandwidth control.
> +	 */
> +	if (cfs_rq->runtime_enabled)
> +		tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED);
> +}
> +#endif

So from a CFS_BANDWIDTH pov runtime_enabled && nr_running == 1 seems
fine. But working around sched_can_stop_tick instead of with it seems
sketchy in general, and in an edge case like "migrate a task onto the
cpu and then off again" you'd get sched_update_tick_dependency resetting
the TICK_DEP_BIT and then not call PNT (ie a task wakes up onto this cpu
without preempting, and then another cpu goes idle and pulls it, causing
this cpu to go into nohz_full).