Messages in this thread Patch in this message | | | Date | Wed, 23 Aug 2023 10:05:31 -0400 | Subject | Re: [PATCH 1/1] sched/fair: ratelimit update to tg->load_avg | From | Mathieu Desnoyers <> |
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On 8/23/23 02:08, 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.
By applying your patch on top of my patchset at:
https://lore.kernel.org/lkml/20230822113133.643238-1-mathieu.desnoyers@efficios.com/
The combined hackbench results look very promising:
(hackbench -g 32 -f 20 --threads --pipe -l 480000 -s 100) (192 cores AMD EPYC 9654 96-Core Processor (over 2 sockets), with hyperthreading)
Baseline: 49s With L2-ttwu-queue-skip: 34s (30% speedup) With L2-ttwu-queue-skip + ratelimit-load-avg: 26s (46% speedup)
Feel free to apply my:
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Tested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Thanks Aaron!
Mathieu
> > ============================== > 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@intel.com> > Suggested-by: Vincent Guittot <vincent.guittot@linaro.org> > Signed-off-by: Aaron Lu <aaron.lu@intel.com> > Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > 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 > + * unbound. Limit the update rate to at most once per ms. > + */ > + 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;
-- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com
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