Re: [PATCH 1/1] sched/fair: ratelimit update to tg->load_avg

[Aaron Lu]

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@xxxxxxxxxxxx/
> 
> 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@xxxxxxxxxxxx>
> Tested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>

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.

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@xxxxxxxxx>
> > Suggested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx>
> > Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > ---
> >   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
>