Re: [PATCH -tip 08/32] sched/fair: Fix forced idle sibling starvation corner case

[Balbir Singh]

On Tue, Nov 17, 2020 at 06:19:38PM -0500, Joel Fernandes (Google) wrote:
> From: Vineeth Pillai <viremana@xxxxxxxxxxxxxxxxxxx>
> 
> 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.
> 
> Tested-by: Julien Desfossez <jdesfossez@xxxxxxxxxxxxxxxx>
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Vineeth Pillai <viremana@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Julien Desfossez <jdesfossez@xxxxxxxxxxxxxxxx>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c  | 15 ++++++++-------
>  kernel/sched/fair.c  | 40 ++++++++++++++++++++++++++++++++++++++++
>  kernel/sched/sched.h |  2 +-
>  3 files changed, 49 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1bd0b0bbb040..52d0e83072a4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5206,16 +5206,15 @@ 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);
>  	}
> @@ -5335,8 +5334,10 @@ 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;
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f53681cd263e..42965c4fd71f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10692,6 +10692,44 @@ 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);

I wonder if the definition of sched_slice() should be revisited for core
scheduling?

Should we use sched_slice = sched_slice / cpumask_weight(smt_mask)?
Would that resolve the issue your seeing? Effectively we need to answer
if two sched core siblings should be treated as executing one large
slice?

Balbir Singh.