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

[Dietmar Eggemann]

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;
+
                return p;
        }