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

From: David Vernet
Date: Thu Aug 24 2023 - 14:48:56 EST

On Wed, Aug 23, 2023 at 02:08:32PM +0800, 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.
>
> ==============================
> 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>

Hey Aaron,

Thanks for working on this. It LGTM modulo two small nits. Feel free to
add my Reviewed-by if you'd like regardless:

Reviewed-by: David Vernet <void@xxxxxxxxxxxxx>

> ---
> 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

s/workload/workloads

> + * unbound. Limit the update rate to at most once per ms.

Can we describe either here or in the commit summary how we arrived at
1ms? I'm fine with hard-coded heuristics like this (just like the
proposed 6-core shard size in the shared_runq patchset), but it would
also be ideal to give a bit more color on how we arrived here, because
we'll forget immediately otherwise.

> + */
> + 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;
> --
> 2.41.0
>