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.l...@hp.com>
> > ---
> > 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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