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sched/fair: Fix forced idle sibling starvation corner case 

If there is only one long running local task and the sibling is
forced idle, it  might not get a chance to run until a schedule
event happens on any cpu in the core.

So we check for this condition during a tick to see if a sibling
is starved and then give it a chance to schedule.

Signed-off-by: Vineeth Pillai <viremana@linux.microsoft.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Don Hiatt <dhiatt@digitalocean.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Tested-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210422123308.617407840@infradead.org
This commit is contained in:
Vineeth Pillai 2020-11-17 18:19:38 -05:00 committed by Peter Zijlstra
parent 539f65125d
commit 8039e96fcc
3 changed files with 49 additions and 8 deletions
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15
kernel/sched/core.c
View File

@ -5459,16 +5459,15 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
/* reset state */
rq->core->core_cookie = 0UL;
if (rq->core->core_forceidle) {
need_sync = true;
rq->core->core_forceidle = false;
}
for_each_cpu(i, smt_mask) {
struct rq *rq_i = cpu_rq(i);
rq_i->core_pick = NULL;
if (rq_i->core_forceidle) {
need_sync = true;
rq_i->core_forceidle = false;
}
if (i != cpu)
update_rq_clock(rq_i);
}
@ -5588,8 +5587,10 @@ next_class:;
if (!rq_i->core_pick)
continue;
if (is_task_rq_idle(rq_i->core_pick) && rq_i->nr_running)
rq_i->core_forceidle = true;
if (is_task_rq_idle(rq_i->core_pick) && rq_i->nr_running &&
!rq_i->core->core_forceidle) {
rq_i->core->core_forceidle = true;
}
if (i == cpu) {
rq_i->core_pick = NULL;

40
kernel/sched/fair.c
View File

@ -10767,6 +10767,44 @@ static void rq_offline_fair(struct rq *rq)
#endif /* CONFIG_SMP */
#ifdef CONFIG_SCHED_CORE
static inline bool
__entity_slice_used(struct sched_entity *se, int min_nr_tasks)
{
u64 slice = sched_slice(cfs_rq_of(se), se);
u64 rtime = se->sum_exec_runtime - se->prev_sum_exec_runtime;
return (rtime * min_nr_tasks > slice);
}
#define MIN_NR_TASKS_DURING_FORCEIDLE 2
static inline void task_tick_core(struct rq *rq, struct task_struct *curr)
{
if (!sched_core_enabled(rq))
return;
/*
* If runqueue has only one task which used up its slice and
* if the sibling is forced idle, then trigger schedule to
* give forced idle task a chance.
*
* sched_slice() considers only this active rq and it gets the
* whole slice. But during force idle, we have siblings acting
* like a single runqueue and hence we need to consider runnable
* tasks on this cpu and the forced idle cpu. Ideally, we should
* go through the forced idle rq, but that would be a perf hit.
* We can assume that the forced idle cpu has atleast
* MIN_NR_TASKS_DURING_FORCEIDLE - 1 tasks and use that to check
* if we need to give up the cpu.
*/
if (rq->core->core_forceidle && rq->cfs.nr_running == 1 &&
__entity_slice_used(&curr->se, MIN_NR_TASKS_DURING_FORCEIDLE))
resched_curr(rq);
}
#else
static inline void task_tick_core(struct rq *rq, struct task_struct *curr) {}
#endif
/*
* scheduler tick hitting a task of our scheduling class.
*
@ -10790,6 +10828,8 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
update_misfit_status(curr, rq);
update_overutilized_status(task_rq(curr));
task_tick_core(rq, curr);
}
/*

2
kernel/sched/sched.h
View File

@ -1083,12 +1083,12 @@ struct rq {
unsigned int core_enabled;
unsigned int core_sched_seq;
struct rb_root core_tree;
unsigned char core_forceidle;
/* shared state */
unsigned int core_task_seq;
unsigned int core_pick_seq;
unsigned long core_cookie;
unsigned char core_forceidle;
#endif
};