linux-kernel - Re: [PATCH v2 sched_ext/for-6.12] sched/fair: Make balance_fair() test sched_fair_runnable() instead of rq->nr

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Message-ID: <6043863c-0343-32b8-8521-3e2fcdbc01fc@amd.com>
Date: Tue, 6 Aug 2024 10:56:42 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Tejun Heo <tj@...nel.org>, Peter Zijlstra <peterz@...radead.org>
CC: Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	<kernel-team@...a.com>, David Vernet <void@...ifault.com>
Subject: Re: [PATCH v2 sched_ext/for-6.12] sched/fair: Make balance_fair()
 test sched_fair_runnable() instead of rq->nr_running

Hello Tejun,

Thank you for updating the commit message.

On 8/6/2024 4:09 AM, Tejun Heo wrote:
> balance_fair() skips newidle balancing if rq->nr_running - there are already
> tasks on the rq, so no need to try to pull tasks. This tests the total
> number of queued tasks on the CPU instead of only the fair class, but is
> still correct as the rq can currently only have fair class tasks while
> balance_fair() is running.
> 
> However, with the addition of sched_ext below the fair class, this will not
> hold anymore and make put_prev_task_balance() skip sched_ext's balance()
> incorrectly as, when a CPU has only lower priority class tasks,
> rq->nr_running would still be positive and balance_fair() would return 1
> even when fair doesn't have any tasks to run.
> 
> Update balance_fair() to use sched_fair_runnable() which tests
> rq->cfs.nr_running which is updated by bandwidth throttling. Note that
> pick_next_task_fair() already uses sched_fair_runnable() in its optimized
> path for the same purpose.
> 
> v2: K Prateek Nayak pointed out that the bw control issue described in v1
>      was incorrect. Patch description updated. As this makes the patch only
>      relevant for sched_ext, I'll carry this through the sched_ext tree
>      unless there are objections.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reported-by: Peter Zijlstra <peterz@...radead.org>
> Cc: K Prateek Nayak <kprateek.nayak@....com>

Looks good to me!

Reviewed-by: K Prateek Nayak <kprateek.nayak@....com>
-- 
Thanks and Regards,
Prateek

> ---
>   kernel/sched/fair.c |    2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8323,7 +8323,7 @@ static void set_cpus_allowed_fair(struct
>   static int
>   balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>   {
> -	if (rq->nr_running)
> +	if (sched_fair_runnable(rq))
>   		return 1;
>   
>   	return sched_balance_newidle(rq, rf) != 0;