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

[Gautham R. Shenoy]

Hello Aaron,
(Adding David Vernet)

On Wed, Aug 16, 2023 at 10:48:31AM +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.


Interestingly I observed this contention on 2 socket EPYC servers
(Zen3 and Zen4) while running tbench and netperf with David Vernet's
shared-runqueue v3 patches. This contention was observed only when
running with the shared-runqueue enabled but not otherwise.

  Overhead  Command  Shared Object     Symbol
+   20.54%  tbench   [kernel.vmlinux]  [k] update_cfs_group
+   15.78%  tbench   [kernel.vmlinux]  [k] update_load_avg

This was causing the tbench (and netperf) to not scale beyond 32
clients when shared-runqueue was enabled.

> 
> 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.

With the shared-runqueue patches, we see a lot more task migrations
since the newidle_balance() path would pull tasks from the
shared-runqueue. While the read of tg->load_avg is via READ_ONCE on
x86, the write is atomic.

> 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.

I will try this patch on with David's series today.

--
Thanks and Regards
gautham.