Messages in this thread | | | Date | Tue, 5 Aug 2014 05:42:04 +0800 | From | Yuyang Du <> | Subject | Re: [PATCH] sched: Reduce contention in update_cfs_rq_blocked_load |
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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@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 > > > > > > > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to majordomo@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/ > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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