linux-kernel - Re: [PATCH 0/2] Fix imbalance issue when balancing fork

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Message-ID: <20250704091758.GG2001818@noisy.programming.kicks-ass.net>
Date: Fri, 4 Jul 2025 11:17:58 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Adam Li <adamli@...amperecomputing.com>
Cc: mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, cl@...ux.com,
	linux-kernel@...r.kernel.org, patches@...erecomputing.com
Subject: Re: [PATCH 0/2] Fix imbalance issue when balancing fork

On Tue, Jul 01, 2025 at 02:45:47AM +0000, Adam Li wrote:
> Load imbalance is observed when the workload frequently forks new threads.
> Due to CPU affinity, the workload can run on CPU 0-7 in the first
> group, and only on CPU 8-11 in the second group. CPU 12-15 are always idle.
> 
> { 0 1 2 3 4 5 6 7 } {8 9 10 11 12 13 14 15}
>   * * * * * * * *    * * *  *
> 
> When looking for dst group for newly forked threads, in many times
> update_sg_wakeup_stats() reports the second group has more idle CPUs
> than the first group. The scheduler thinks the second group is less
> busy. Then it selects least busy CPUs among CPU 8-11. So CPU 8-11 can be
> crowded with newly forked threads, at the same time CPU 0-7 can be idle.
> 
> The first patch 'Only update stats of allowed CPUs when looking for dst
> group' *alone* can fix this imbalance issue.
> 
> And I think the second patch also makes sense in this scenario. If group
> weight includes CPUs a task cannot use, group classification can be
> incorrect. Please comment.
> 
> Adam Li (2):
>   sched/fair: Only update stats of allowed CPUs when looking for dst
>     group
>   sched/fair: Only count group weight for allowed CPUs when looking for
>     dst group
> 
>  kernel/sched/fair.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 

Hurm... so the thing I noticed is that update_sg_wakeup_stats() and
update_sg_lb_stats() are *very* similar.

Specifically, the first patch does something to wakeup_stats that
lb_stats already does. While the second patch seems to do something that
might also apply to lb_stats.

Is there no way we can unify this?