Re: [RFC][PATCH 12/13] sched/deadline: Introduce deadline servers

[Juri Lelli]

On 08/08/19 10:11, Dietmar Eggemann wrote:
> On 8/8/19 9:56 AM, Peter Zijlstra wrote:
> > On Wed, Aug 07, 2019 at 06:31:59PM +0200, Dietmar Eggemann wrote:
> >> On 7/26/19 4:54 PM, Peter Zijlstra wrote:
> >>>
> >>>
> >>> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> >>
> >> [...]
> >>
> >>> @@ -889,6 +891,8 @@ static void update_curr(struct cfs_rq *c
> >>>  		trace_sched_stat_runtime(curtask, delta_exec, curr->vruntime);
> >>>  		cgroup_account_cputime(curtask, delta_exec);
> >>>  		account_group_exec_runtime(curtask, delta_exec);
> >>> +		if (curtask->server)
> >>> +			dl_server_update(curtask->server, delta_exec);
> >>>  	}
> >>
> >> I get a lockdep_assert_held(&rq->lock) related warning in start_dl_timer()
> >> when running the full stack.
> > 
> > That would seem to imply a stale curtask->server value; the hunk below:
> > 
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3756,8 +3756,11 @@ pick_next_task(struct rq *rq, struct tas
> > 
> >         for_each_class(class) {
> >                 p = class->pick_next_task(rq, NULL, NULL);
> > -               if (p)
> > +               if (p) {
> > +                       if (p->sched_class == class && p->server)
> > +                               p->server = NULL;
> >                         return p;
> > +               }
> >         }
> > 
> > 
> > Was supposed to clear p->server, but clearly something is going 'funny'.
> 
> What about the fast path in pick_next_task()?
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bffe849b5a42..f1ea6ae16052 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3742,6 +3742,9 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>                 if (unlikely(!p))
>                         p = idle_sched_class.pick_next_task(rq, prev, rf);
>  
> +               if (p->sched_class == &fair_sched_class && p->server)
> +                       p->server = NULL;
> +

Hummm, but then who sets it back to the correct server. AFAIU
update_curr() needs a ->server to do the correct DL accounting?