linux-kernel - Re: [PATCH] sched/fair: Make cfs_rq::decay

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Date:	Wed, 17 Dec 2014 12:09:33 +0300
From:	Kirill Tkhai <ktkhai@...allels.com>
To:	<bsegall@...gle.com>
CC:	<linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Kirill Tkhai <tkhai@...dex.ru>
Subject: Re: [PATCH] sched/fair: Make cfs_rq::decay_counter non-atomic

В Вт, 16/12/2014 в 10:00 -0800, bsegall@...gle.com пишет:
> Kirill Tkhai <ktkhai@...allels.com> writes:
> 
> > We update decay_counter in update_cfs_rq_blocked_load()
> > only. This function is always called with rq lock locked,
> > so we can kill atomic actions.
> >
> > Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
> > ---
> >  kernel/sched/fair.c  |   11 ++++++-----
> >  kernel/sched/sched.h |    2 +-
> >  2 files changed, 7 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 5f3b5a7..af990c4 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -2570,7 +2570,7 @@ static __always_inline int __update_entity_runnable_avg(u64 now,
> >  static inline u64 __synchronize_entity_decay(struct sched_entity *se)
> >  {
> >  	struct cfs_rq *cfs_rq = cfs_rq_of(se);
> > -	u64 decays = atomic64_read(&cfs_rq->decay_counter);
> > +	u64 decays = ACCESS_ONCE(cfs_rq->decay_counter);
> 
> This is called without rq lock held from migrate_task_rq_fair. (We could
> technically change the atomic_add to atomic64_set(atomic64_read() + x),
> but I don't know that that is a win) Now, we could do a
> min_vruntime-style two-copy thing if this atomic usage is a hot spot on
> 32-bit, we just didn't bother initially.

Oh, I forgot that 64-bit read is not everywhere atomic... Thanks, Ben.

Peter, please, ignore this patch.

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