Re: PELT initial task load and wake_up_new_task()

[Yuyang Du]

On Thu, Dec 17, 2015 at 10:43:03AM +0100, Peter Zijlstra wrote:
> On Thu, Dec 17, 2015 at 07:34:27AM +0800, Yuyang Du wrote:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index e3266eb..3f6a8b3 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -2908,10 +2908,18 @@ void remove_entity_load_avg(struct sched_entity *se)
> >  {
> >  	struct cfs_rq *cfs_rq = cfs_rq_of(se);
> >  	u64 last_update_time;
> > -
> >  #ifndef CONFIG_64BIT
> >  	u64 last_update_time_copy;
> > +#endif
> >  
> > +	/*
> > +	 * Newly created task or never used group entity should not be removed
> > +	 * from its (source) cfs_rq
> > +	 */
> > +	if (se->avg.last_update_time == 0)
> > +		return;
> > +
> > +#ifndef CONFIG_64BIT
> >  	do {
> >  		last_update_time_copy = cfs_rq->load_last_update_time_copy;
> >  		smp_rmb();
> 
> So that ifdef stuff annoyed me a wee bit, so I did the below.
> 
> Initially I wanted to do a macro that we could use for both this and the
> vruntime, but that didn't end up particularly pretty either, so I
> scrapped that.

Oh yeah, looks good. :)
 
> ---
> Subject: sched: Fix new task's load avg removed from source CPU in wake_up_new_task()
> From: Yuyang Du <yuyang.du@xxxxxxxxx>
> Date: Thu, 17 Dec 2015 07:34:27 +0800
> 
> If a newly created task is selected to go to a different CPU in fork
> balance when it wakes up the first time, its load averages should
> not be removed from the source CPU since they are never added to
> it before. The same is also applicable to a never used group entity.
> 
> Fix it in remove_entity_load_avg(): when entity's last_update_time
> is 0, simply return. This should precisely identify the case in
> question, because in other migrations, the last_update_time is set
> to 0 after remove_entity_load_avg().
> 
> Reported-by: Steve Muckle <steve.muckle@xxxxxxxxxx>
> Signed-off-by: Yuyang Du <yuyang.du@xxxxxxxxx>
> Cc: Patrick Bellasi <patrick.bellasi@xxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Juri Lelli <Juri.Lelli@xxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Morten Rasmussen <morten.rasmussen@xxxxxxx>
> [peterz: cfs_rq_last_update_time]
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Link: http://lkml.kernel.org/r/20151216233427.GJ28098@xxxxxxxxx
> ---
>  kernel/sched/fair.c |   38 ++++++++++++++++++++++++++++----------
>  1 file changed, 28 insertions(+), 10 deletions(-)
> 
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2809,27 +2809,45 @@ dequeue_entity_load_avg(struct cfs_rq *c
>  		max_t(s64,  cfs_rq->runnable_load_sum - se->avg.load_sum, 0);
>  }
>  
> -/*
> - * Task first catches up with cfs_rq, and then subtract
> - * itself from the cfs_rq (task must be off the queue now).
> - */
> -void remove_entity_load_avg(struct sched_entity *se)
> -{
> -	struct cfs_rq *cfs_rq = cfs_rq_of(se);
> -	u64 last_update_time;
> -
>  #ifndef CONFIG_64BIT
> +static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
> +{
>  	u64 last_update_time_copy;
> +	u64 last_update_time;
>  
>  	do {
>  		last_update_time_copy = cfs_rq->load_last_update_time_copy;
>  		smp_rmb();
>  		last_update_time = cfs_rq->avg.last_update_time;
>  	} while (last_update_time != last_update_time_copy);
> +
> +	return last_update_time;
> +}
>  #else
> -	last_update_time = cfs_rq->avg.last_update_time;
> +static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
> +{
> +	return cfs_rq->avg.last_update_time;
> +}
>  #endif
>  
> +/*
> + * Task first catches up with cfs_rq, and then subtract
> + * itself from the cfs_rq (task must be off the queue now).
> + */
> +void remove_entity_load_avg(struct sched_entity *se)
> +{
> +	struct cfs_rq *cfs_rq = cfs_rq_of(se);
> +	u64 last_update_time;
> +
> +	/*
> +	 * Newly created task or never used group entity should not be removed
> +	 * from its (source) cfs_rq
> +	 */
> +	if (se->avg.last_update_time == 0)
> +		return;
> +
> +	last_update_time = cfs_rq_last_update_time(cfs_rq);
> +
>  	__update_load_avg(last_update_time, cpu_of(rq_of(cfs_rq)), &se->avg, 0, 0, NULL);
>  	atomic_long_add(se->avg.load_avg, &cfs_rq->removed_load_avg);
>  	atomic_long_add(se->avg.util_avg, &cfs_rq->removed_util_avg);
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