Re: [PATCH 1/1] sched/fair: ratelimit update to tg->load_avg
From: Aaron Lu
Date: Fri Aug 25 2023 - 02:19:51 EST
On Thu, Aug 24, 2023 at 01:48:07PM -0500, David Vernet wrote:
> On Wed, Aug 23, 2023 at 02:08:32PM +0800, Aaron Lu wrote:
> > When using sysbench to benchmark Postgres in a single docker instance
> > with sysbench's nr_threads set to nr_cpu, it is observed there are times
> > update_cfs_group() and update_load_avg() shows noticeable overhead on
> > a 2sockets/112core/224cpu Intel Sapphire Rapids(SPR):
> >
> > 13.75% 13.74% [kernel.vmlinux] [k] update_cfs_group
> > 10.63% 10.04% [kernel.vmlinux] [k] update_load_avg
> >
> > Annotate shows the cycles are mostly spent on accessing tg->load_avg
> > with update_load_avg() being the write side and update_cfs_group() being
> > the read side. tg->load_avg is per task group and when different tasks
> > of the same taskgroup running on different CPUs frequently access
> > tg->load_avg, it can be heavily contended.
> >
> > E.g. when running postgres_sysbench on a 2sockets/112cores/224cpus Intel
> > Sappire Rapids, during a 5s window, the wakeup number is 14millions and
> > migration number is 11millions and with each migration, the task's load
> > will transfer from src cfs_rq to target cfs_rq and each change involves
> > an update to tg->load_avg. Since the workload can trigger as many wakeups
> > and migrations, the access(both read and write) to tg->load_avg can be
> > unbound. As a result, the two mentioned functions showed noticeable
> > overhead. With netperf/nr_client=nr_cpu/UDP_RR, the problem is worse:
> > during a 5s window, wakeup number is 21millions and migration number is
> > 14millions; update_cfs_group() costs ~25% and update_load_avg() costs ~16%.
> >
> > Reduce the overhead by limiting updates to tg->load_avg to at most once
> > per ms. After this change, the cost of accessing tg->load_avg is greatly
> > reduced and performance improved. Detailed test results below.
> >
> > ==============================
> > postgres_sysbench on SPR:
> > 25%
> > base: 42382±19.8%
> > patch: 50174±9.5% (noise)
> >
> > 50%
> > base: 67626±1.3%
> > patch: 67365±3.1% (noise)
> >
> > 75%
> > base: 100216±1.2%
> > patch: 112470±0.1% +12.2%
> >
> > 100%
> > base: 93671±0.4%
> > patch: 113563±0.2% +21.2%
> >
> > ==============================
> > hackbench on ICL:
> > group=1
> > base: 114912±5.2%
> > patch: 117857±2.5% (noise)
> >
> > group=4
> > base: 359902±1.6%
> > patch: 361685±2.7% (noise)
> >
> > group=8
> > base: 461070±0.8%
> > patch: 491713±0.3% +6.6%
> >
> > group=16
> > base: 309032±5.0%
> > patch: 378337±1.3% +22.4%
> >
> > =============================
> > hackbench on SPR:
> > group=1
> > base: 100768±2.9%
> > patch: 103134±2.9% (noise)
> >
> > group=4
> > base: 413830±12.5%
> > patch: 378660±16.6% (noise)
> >
> > group=8
> > base: 436124±0.6%
> > patch: 490787±3.2% +12.5%
> >
> > group=16
> > base: 457730±3.2%
> > patch: 680452±1.3% +48.8%
> >
> > ============================
> > netperf/udp_rr on ICL
> > 25%
> > base: 114413±0.1%
> > patch: 115111±0.0% +0.6%
> >
> > 50%
> > base: 86803±0.5%
> > patch: 86611±0.0% (noise)
> >
> > 75%
> > base: 35959±5.3%
> > patch: 49801±0.6% +38.5%
> >
> > 100%
> > base: 61951±6.4%
> > patch: 70224±0.8% +13.4%
> >
> > ===========================
> > netperf/udp_rr on SPR
> > 25%
> > base: 104954±1.3%
> > patch: 107312±2.8% (noise)
> >
> > 50%
> > base: 55394±4.6%
> > patch: 54940±7.4% (noise)
> >
> > 75%
> > base: 13779±3.1%
> > patch: 36105±1.1% +162%
> >
> > 100%
> > base: 9703±3.7%
> > patch: 28011±0.2% +189%
> >
> > ==============================================
> > netperf/tcp_stream on ICL (all in noise range)
> > 25%
> > base: 43092±0.1%
> > patch: 42891±0.5%
> >
> > 50%
> > base: 19278±14.9%
> > patch: 22369±7.2%
> >
> > 75%
> > base: 16822±3.0%
> > patch: 17086±2.3%
> >
> > 100%
> > base: 18216±0.6%
> > patch: 18078±2.9%
> >
> > ===============================================
> > netperf/tcp_stream on SPR (all in noise range)
> > 25%
> > base: 34491±0.3%
> > patch: 34886±0.5%
> >
> > 50%
> > base: 19278±14.9%
> > patch: 22369±7.2%
> >
> > 75%
> > base: 16822±3.0%
> > patch: 17086±2.3%
> >
> > 100%
> > base: 18216±0.6%
> > patch: 18078±2.9%
> >
> > Reported-by: Nitin Tekchandani <nitin.tekchandani@xxxxxxxxx>
> > Suggested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx>
> > Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>
> Hey Aaron,
>
> Thanks for working on this. It LGTM modulo two small nits. Feel free to
> add my Reviewed-by if you'd like regardless:
>
> Reviewed-by: David Vernet <void@xxxxxxxxxxxxx>
Thanks!
> > ---
> > kernel/sched/fair.c | 13 ++++++++++++-
> > kernel/sched/sched.h | 1 +
> > 2 files changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index c28206499a3d..a5462d1fcc48 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -3664,7 +3664,8 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
> > */
> > static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
> > {
> > - long delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
> > + long delta;
> > + u64 now;
> >
> > /*
> > * No need to update load_avg for root_task_group as it is not used.
> > @@ -3672,9 +3673,19 @@ static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
> > if (cfs_rq->tg == &root_task_group)
> > return;
> >
> > + /*
> > + * For migration heavy workload, access to tg->load_avg can be
>
> s/workload/workloads
Will change.
> > + * unbound. Limit the update rate to at most once per ms.
>
> Can we describe either here or in the commit summary how we arrived at
> 1ms? I'm fine with hard-coded heuristics like this (just like the
> proposed 6-core shard size in the shared_runq patchset), but it would
> also be ideal to give a bit more color on how we arrived here, because
> we'll forget immediately otherwise.
Agree. As I replied to Mathieu, I chose 1ms mainly because pelt window
is roughly 1ms. I'll update the changelog when sending a new version.
Thanks,
Aaron
> > + */
> > + now = sched_clock_cpu(cpu_of(rq_of(cfs_rq)));
> > + if (now - cfs_rq->last_update_tg_load_avg < NSEC_PER_MSEC)
> > + return;
> > +
> > + delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
> > if (abs(delta) > cfs_rq->tg_load_avg_contrib / 64) {
> > atomic_long_add(delta, &cfs_rq->tg->load_avg);
> > cfs_rq->tg_load_avg_contrib = cfs_rq->avg.load_avg;
> > + cfs_rq->last_update_tg_load_avg = now;
> > }
> > }
> >
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 6a8b7b9ed089..52ee7027def9 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -593,6 +593,7 @@ struct cfs_rq {
> > } removed;
> >
> > #ifdef CONFIG_FAIR_GROUP_SCHED
> > + u64 last_update_tg_load_avg;
> > unsigned long tg_load_avg_contrib;
> > long propagate;
> > long prop_runnable_sum;
> > --
> > 2.41.0
> >