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

[Aaron Lu <aaron.lu@intel.com>]

On Thu, Aug 17, 2023 at 02:56:00PM +0200, Vincent Guittot wrote:
> On Wed, 16 Aug 2023 at 04:48, Aaron Lu <aaron.lu@intel.com> 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.
> >
> > The frequent access to tg->load_avg is due to task migration on wakeup
> > path, 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.
> >
> > ==============================
> > 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%
> >
> > Signed-off-by: Aaron Lu <aaron.lu@intel.com>
> > ---
> >  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 6e79de26a25d..ab055c72cc64 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 19af1766df2d..228a298bf3b5 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -594,6 +594,7 @@ struct cfs_rq {
> >
> >  #ifdef CONFIG_FAIR_GROUP_SCHED
> >         unsigned long           tg_load_avg_contrib;
> > +       u64                     last_update_tg_load_avg;
> 
> Moving last_update_tg_load_avg before tg_load_avg_contrib should
> minimize the padding on 32bits arch as long is only 4 Bytes

Got it. That would be:

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;
-- 
2.41.0

> 
> Apart from this, looks good to me

Thanks a lot for your review!
Can I add your reviewed-by in my next update with the above change?

Regards,
Aaron

> >         long                    propagate;
> >         long                    prop_runnable_sum;
> >
> > --
> > 2.41.0
> >