Re: [PATCH v3 4/5] sched: sync a se with its cfs_rq when switching sched class to fair class

Peter Zijlstra
2015-08-20T21:13:29Z

On Thu, Aug 20, 2015 at 04:11:06AM +0800, Yuyang Du wrote:
> On Wed, Aug 19, 2015 at 07:12:41PM +0200, Peter Zijlstra wrote:
> > On Wed, Aug 19, 2015 at 03:47:15PM +0900, byungchul.park@lge.com wrote:
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 1be042a..3419f6c 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -2711,6 +2711,17 @@ static inline void update_load_avg(struct sched_entity *se, int update_tg)
> > >
> > > static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > > {
> > > + /*
> > > + * in case of migration and cgroup-change, more care should be taken
> > > + * because se's cfs_rq was changed, that means calling __update_load_avg
> > > + * with new cfs_rq->avg.last_update_time is meaningless. so we skip the
> > > + * update here. we have to update it with prev cfs_rq just before changing
> > > + * se's cfs_rq, and get here soon.
> > > + */
> > > + if (se->avg.last_update_time)
> > > + __update_load_avg(cfs_rq->avg.last_update_time, cpu_of(rq_of(cfs_rq)),
> > > + &se->avg, 0, 0, NULL);
> > > +
> > > se->avg.last_update_time = cfs_rq->avg.last_update_time;
> > > cfs_rq->avg.load_avg += se->avg.load_avg;
> > > cfs_rq->avg.load_sum += se->avg.load_sum;
> >
> > you seem to have forgotten to remove the same logic from
> > enqueue_entity_load_avg(), which will now call __update_load_avg()
> > twice.
>
> In case of enqueue_entity_load_avg(), that seems to be ok.
>
> However, the problem is that he made it "entangled":
>
> In enqueue_entity_load_avg():
>
> if (migrated)
> attach_entity_load_avg();
>
> while in attach_entity_load_avg():
>
> if (!migrated)
> __update_load_avg();
>
> so, if attach() is called from enqueue(), that if() is never true.

Right, I noticed the same yesterday when I took a second look at that
stuff. It was a little confusing indeed.