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Date:	Thu, 28 Aug 2014 12:46:36 -0700
From:	Jason Low <jason.low2@...com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Paul Turner <pjt@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Ben Segall <bsegall@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sched: Reduce contention in
 update_cfs_rq_blocked_load

On Wed, 2014-08-27 at 16:32 -0700, Tim Chen wrote:
> On Wed, 2014-08-27 at 10:34 -0700, Jason Low wrote:
> > On Tue, 2014-08-26 at 16:24 -0700, Paul Turner wrote:
> > > On Tue, Aug 26, 2014 at 4:11 PM, Jason Low <jason.low2@...com> wrote:
> > > > Based on perf profiles, the update_cfs_rq_blocked_load function constantly
> > > > shows up as taking up a noticeable % of system run time. This is especially
> > > > apparent on larger numa systems.
> > > >
> > > > Much of the contention is in __update_cfs_rq_tg_load_contrib when we're
> > > > updating the tg load contribution stats. However, it was noticed that the
> > > > values often don't get modified by much. In fact, much of the time, they
> > > > don't get modified at all. However, the update can always get attempted due
> > > > to force_update.
> > > >
> > > > In this patch, we remove the force_update in only the
> > > > __update_cfs_rq_tg_load_contrib. Thus the tg load contrib stats now get
> > > > modified only if the delta is large enough (in the current code, they get
> > > > updated when the delta is larger than 12.5%). This is a way to rate-limit
> > > > the updates while largely keeping the values accurate.
> > > >
> > > > When testing this change with AIM7 workloads, we found that it was able to
> > > > reduce the overhead of the function by up to a factor of 20x.
> > > 
> > > Looks reasonable.
> > > 
> > > >
> > > > Cc: Yuyang Du <yuyang.du@...el.com>
> > > > Cc: Waiman Long <Waiman.Long@...com>
> > > > Cc: Mel Gorman <mgorman@...e.de>
> > > > Cc: Mike Galbraith <umgwanakikbuti@...il.com>
> > > > Cc: Rik van Riel <riel@...hat.com>
> > > > Cc: Aswin Chandramouleeswaran <aswin@...com>
> > > > Cc: Chegu Vinod <chegu_vinod@...com>
> > > > Cc: Scott J Norton <scott.norton@...com>
> > > > Signed-off-by: Jason Low <jason.low2@...com>
> > > > ---
> > > >  kernel/sched/fair.c |   10 ++++------
> > > >  1 files changed, 4 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > index fea7d33..7a6e18b 100644
> > > > --- a/kernel/sched/fair.c
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -2352,8 +2352,7 @@ static inline u64 __synchronize_entity_decay(struct sched_entity *se)
> > > >  }
> > > >
> > > >  #ifdef CONFIG_FAIR_GROUP_SCHED
> > > > -static inline void __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq,
> > > > -                                                int force_update)
> > > > +static inline void __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq)
> > > >  {
> > > >         struct task_group *tg = cfs_rq->tg;
> > > >         long tg_contrib;
> > > > @@ -2361,7 +2360,7 @@ static inline void __update_cfs_rq_tg_load_contrib(struct cfs_rq *cfs_rq,
> > > >         tg_contrib = cfs_rq->runnable_load_avg + cfs_rq->blocked_load_avg;
> > > >         tg_contrib -= cfs_rq->tg_load_contrib;
> > > >
> > > > -       if (force_update || abs(tg_contrib) > cfs_rq->tg_load_contrib / 8) {
> > > 
> > > Another option with slightly higher accuracy would be to increase the
> > > sensitivity here when force_update == 1.
> > > 
> > > E.g.:
> > >     abs(tg_contrib) > cfs_rq->tg_load_contrib / (8 * (1 + force_update))) { ...
> > > 
> > > Alternatively we could bound total inaccuracy:
> > >    int divisor = force_update ? NR_CPUS : 8;
> > >    if (abs(tg_contrib) > cfs_rq->tg_load_contrib / divisor) { ...
> > > 
> > > 
> > > [ And probably rename force_update to want_update ]
> > 
> > Out of the 2 additional options, I think the first one is better. The
> > other option of using NR_CPUS looks like we're increasing the update
> > rate as the system gets larger, and its the larger systems that are
> > typically more affected by the contention.
> 
> Probably num_present_cpus() will be better than NR_CPUS, which can
> be much larger than the actual cpus present.

Yeah, num_present_cpus(), though the same issue would still be there.

> > 
> > Do you prefer either of the 2 other options over this v2 patch? If so, I
> > can test and send out a new patch, otherwise, I'll keep this current v2
> > patch.
> 
> If there are multiple non-forced updates, option 1's error seems to
> accumulate and non-bounded as we do not actually update?  
> Is this a concern?

It should be fine. Once the delta is large enough, we will end up doing
the update anyway.

Thanks.

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