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Date: Thu, 24 Aug 2023 08:56:40 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Aaron Lu <aaron.lu@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Ingo Molnar <mingo@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>,
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 8/24/23 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@...icios.com>
>> Tested-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.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@...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
>>
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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