linux-kernel - Re: [PATCH] sched: Consider capacity for certain load balancing decisions

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Date:   Fri, 3 Feb 2023 10:51:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Xi Wang <xii@...gle.com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Ben Segall <bsegall@...gle.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] sched: Consider capacity for certain load balancing
 decisions

On Tue, Jan 31, 2023 at 05:20:32PM -0800, Xi Wang wrote:
> After load balancing was split into different scenarios, CPU capacity
> is ignored for the "migrate_task" case, which means a thread can stay
> on a softirq heavy cpu for an extended amount of time.
> 
> By comparing nr_running/capacity instead of just nr_running we can add
> CPU capacity back into "migrate_task" decisions. This benefits
> workloads running on machines with heavy network traffic. The change
> is unlikely to cause serious problems for other workloads but maybe
> some corner cases still need to be considered.
> 
> Signed-off-by: Xi Wang <xii@...gle.com>
> ---
>  kernel/sched/fair.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0f8736991427..aad14bc04544 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10368,8 +10368,9 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>  			break;
>  
>  		case migrate_task:
> -			if (busiest_nr < nr_running) {
> +			if (busiest_nr * capacity < nr_running * busiest_capacity) {
>  				busiest_nr = nr_running;
> +				busiest_capacity = capacity;
>  				busiest = rq;
>  			}
>  			break;

I don't think this is correct. The migrate_task case is work-conserving,
and your change can severely break that I think.