Re: [PATCH] sched: Fix SCHED_HRTICK bug leading to late preemption of tasks

[Joonwoo Park]

On Sun, Sep 18, 2016 at 07:28:32AM +0800, Wanpeng Li wrote:
> 2016-09-17 9:28 GMT+08:00 Joonwoo Park <joonwoop@xxxxxxxxxxxxxx>:
> > From: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxx>
> >
> > SCHED_HRTICK feature is useful to preempt SCHED_FAIR tasks on-the-dot
> > (just when they would have exceeded their ideal_runtime). It makes use
> > of a per-cpu hrtimer resource and hence alarming that hrtimer should
> > be based on total SCHED_FAIR tasks a cpu has across its various cfs_rqs,
> > rather than being based on number of tasks in a particular cfs_rq (as
> > implemented currently). As a result, with current code, its possible for
> > a running task (which is the sole task in its cfs_rq) to be preempted
> 
> not be preempted much, right?

I don't think so....
By saying 'to be preempted much after its ideal_runtime has elapsed' I
wanted to describe the current suboptimal behaviour.

Thanks,
Joonwoo

> 
> > much after its ideal_runtime has elapsed, resulting in increased latency
> > for tasks in other cfs_rq on same cpu.
> >
> > Fix this by alarming sched hrtimer based on total number of SCHED_FAIR
> > tasks a CPU has across its various cfs_rqs.
> >
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Signed-off-by: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxx>
> > Signed-off-by: Joonwoo Park <joonwoop@xxxxxxxxxxxxxx>
> > ---
> >
> >  joonwoop: Do we also need to update or remove if-statement inside
> >  hrtick_update()?
> >  I guess not because hrtick_update() doesn't want to start hrtick when cfs_rq
> >  has large number of nr_running where slice is longer than sched_latency.
> >
> >  kernel/sched/fair.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 4088eed..c55c566 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4458,7 +4458,7 @@ static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
> >
> >         WARN_ON(task_rq(p) != rq);
> >
> > -       if (cfs_rq->nr_running > 1) {
> > +       if (rq->cfs.h_nr_running > 1) {
> >                 u64 slice = sched_slice(cfs_rq, se);
> >                 u64 ran = se->sum_exec_runtime - se->prev_sum_exec_runtime;
> >                 s64 delta = slice - ran;
> > --
> > The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> > hosted by The Linux Foundation
> >