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Message-ID: <460a9efb-f5f1-614e-1902-45f8500925fd@efficios.com>
Date: Wed, 23 Aug 2023 10:17:56 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Aaron Lu <aaron.lu@...el.com>,
Peter Zijlstra <peterz@...radead.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Ingo Molnar <mingo@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>
Cc: Daniel Jordan <daniel.m.jordan@...cle.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
Tim Chen <tim.c.chen@...el.com>,
Nitin Tekchandani <nitin.tekchandani@...el.com>,
Yu Chen <yu.c.chen@...el.com>,
Waiman Long <longman@...hat.com>,
Deng Pan <pan.deng@...el.com>,
"Gautham R . Shenoy" <gautham.shenoy@....com>,
David Vernet <void@...ifault.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] sched/fair: ratelimit update to tg->load_avg
On 8/23/23 10:05, 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)
Here is an additional interesting data point:
With only ratelimit-load-avg patch: 32s (35% speedup)
So each series appear to address a different scalability issue, and
combining both seems worthwhile, at least from the point of view of
this specific benchmark on this hardware.
I'm looking forward to see numbers for other benchmarks and hardware.
Thanks,
Mathieu
>
> Feel free to apply my:
>
> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Tested-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.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@...el.com>
>> Suggested-by: Vincent Guittot <vincent.guittot@...aro.org>
>> Signed-off-by: Aaron Lu <aaron.lu@...el.com>
>> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.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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