sched/fair: Limit sched slice duration

In presence of a lot of small weight tasks like sched_idle tasks, normal
or high weight tasks can see their ideal runtime (sched_slice) to increase
to hundreds ms whereas it normally stays below sysctl_sched_latency.

2 normal tasks running on a CPU will have a max sched_slice of 12ms
(half of the sched_period). This means that they will make progress
every sysctl_sched_latency period.

If we now add 1000 idle tasks on the CPU, the sched_period becomes
3006 ms and the ideal runtime of the normal tasks becomes 609 ms.
It will even become 1500ms if the idle tasks belongs to an idle cgroup.
This means that the scheduler will look for picking another waiting task
after 609ms running time (1500ms respectively). The idle tasks change
significantly the way the 2 normal tasks interleave their running time
slot whereas they should have a small impact.

Such long sched_slice can delay significantly the release of resources
as the tasks can wait hundreds of ms before the next running slot just
because of idle tasks queued on the rq.

Cap the ideal_runtime to sysctl_sched_latency to make sure that tasks will
regularly make progress and will not be significantly impacted by
idle/background tasks queued on the rq.

Signed-off-by: Vincent Guittot <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Tested-by: Dietmar Eggemann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

kernel/sched/fair.c

···
 	struct sched_entity *se;
 	s64 delta;
 
-	ideal_runtime = sched_slice(cfs_rq, curr);
+	/*
+	 * When many tasks blow up the sched_period; it is possible that
+	 * sched_slice() reports unusually large results (when many tasks are
+	 * very light for example). Therefore impose a maximum.
+	 */
+	ideal_runtime = min_t(u64, sched_slice(cfs_rq, curr), sysctl_sched_latency);
+
 	delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
 	if (delta_exec > ideal_runtime) {
 		resched_curr(rq_of(cfs_rq));