Re: [PATCH v2 1/1] sched/fair: ratelimit update to tg->load_avg

[Peter Zijlstra]

On Tue, Sep 12, 2023 at 02:58:08PM +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. The update frequency is a tradeoff between tracking accuracy and
> overhead. 1ms is chosen because PELT window is roughly 1ms and it
> delivered good results for the tests that I've done. After this change,
> the cost of accessing tg->load_avg is greatly reduced and performance
> improved. Detailed test results below.
> 

> 
> 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>
> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Tested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Reviewed-by: David Vernet <void@xxxxxxxxxxxxx>
> Tested-by: Swapnil Sapkal <Swapnil.Sapkal@xxxxxxx>

Thanks!