Messages in this thread | | | Date | Thu, 24 Aug 2023 08:56:40 -0400 | Subject | Re: [PATCH 1/1] sched/fair: ratelimit update to tg->load_avg | From | Mathieu Desnoyers <> |
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On 8/24/23 04:01, Aaron Lu wrote: > On Wed, Aug 23, 2023 at 10:05:31AM -0400, Mathieu Desnoyers wrote: >> 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 a lot for running this and reviewing the patch. > I'll add your number and tag in the changelog when sending a new > version.
Now that I come to think of it, I have comment: why use sched_clock_cpu() rather than just read the jiffies value ? AFAIR, sched_clock can be slower than needed when read from a "remote" cpu on architectures that have an unsynchronized tsc.
Considering that you only need a time reference more or less accurate at the millisecond level, I suspect that jiffies is what you are looking for here. This is what the NUMA balance code and rseq mm_cid use to execute work every N milliseconds.
Thanks,
Mathieu
> > Regards, > Aaron > >>> >>> ============================== >>> 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 >>
-- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com
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