Re: [PATCH] sched: Reduce contention in update_cfs_rq_blocked_load

[Yuyang Du]

On Tue, Aug 05, 2014 at 03:15:26AM +0800, Yuyang Du wrote:
> Hi Jason,
> 
> I am not sure whether you noticed my latest work: rewriting per entity load average
> 
> http://article.gmane.org/gmane.linux.kernel/1760754
> http://article.gmane.org/gmane.linux.kernel/1760755
> http://article.gmane.org/gmane.linux.kernel/1760757
> http://article.gmane.org/gmane.linux.kernel/1760756
> 
> which simply does not track blocked load average at all.

Actually, it tracks blocked load, but along with runnable load, so no this extra
overhead.

> Are you interested in
> testing the patchset with the workload you have? The comparison can also help
> us understand the rewrite. Overall, per our tests, the overhead should be less,
> and perf should be better.
> 
> 
> On Mon, Aug 04, 2014 at 01:28:38PM -0700, Jason Low wrote:
> > When running workloads on 2+ socket systems, based on perf profiles, the
> > update_cfs_rq_blocked_load function constantly shows up as taking up a
> > noticeable % of run time. This is especially apparent on an 8 socket
> > machine. For example, when running the AIM7 custom workload, we see:
> > 
> >    4.18%        reaim  [kernel.kallsyms]        [k] update_cfs_rq_blocked_load
> > 
> > Much of the contention is in __update_cfs_rq_tg_load_contrib when we
> > update the tg load contribution stats.  However, it turns out that in many
> > cases, they don't need to be updated and "tg_contrib" is 0.
> > 
> > This patch adds a check in __update_cfs_rq_tg_load_contrib to skip updating
> > tg load contribution stats when nothing needs to be updated. This reduces the
> > cacheline contention that would be unnecessary. In the above case, with the
> > patch, perf reports the total time spent in this function went down by more
> > than a factor of 3x:
> > 
> >    1.18%        reaim  [kernel.kallsyms]        [k] update_cfs_rq_blocked_load
> > 
> > Signed-off-by: Jason Low <jason.low2@xxxxxx>
> > ---
> >  kernel/sched/fair.c |    3 +++
> >  1 files changed, 3 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index bfa3c86..8d4cc72 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -2377,6 +2377,9 @@ 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 (!tg_contrib)
> > +		return;
> > +
> >  	if (force_update || abs(tg_contrib) > cfs_rq->tg_load_contrib / 8) {
> >  		atomic_long_add(tg_contrib, &tg->load_avg);
> >  		cfs_rq->tg_load_contrib += tg_contrib;
> > -- 
> > 1.7.1
> > 
> > 
> > 
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