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From: Benjamin Segall <bsegall@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@redhat.com,  juri.lelli@redhat.com,  vincent.guittot@linaro.org,
  dietmar.eggemann@arm.com,  rostedt@goodmis.org,  mgorman@suse.de,
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  kprateek.nayak@amd.com,  wuyun.abel@bytedance.com,  tglx@linutronix.de,
  efault@gmx.de
Subject: Re: [RFC][PATCH 03/10] sched/fair: Cleanup pick_task_fair() vs
 throttle
In-Reply-To: <20240405110010.024746026@infradead.org> (Peter Zijlstra's
	message of "Fri, 05 Apr 2024 12:27:57 +0200")
References: <20240405102754.435410987@infradead.org>
	<20240405110010.024746026@infradead.org>
Date: Fri, 05 Apr 2024 14:11:57 -0700
Message-ID: <xm26zfu7ecpu.fsf@google.com>
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Peter Zijlstra <peterz@infradead.org> writes:

> Per 54d27365cae8 ("sched/fair: Prevent throttling in early
> pick_next_task_fair()") the reason check_cfs_rq_runtime() is under the
> 'if (curr)' check is to ensure the (downward) traversal does not
> result in an empty cfs_rq.
>
> But then the pick_task_fair() 'copy' of all this made it restart the
> traversal anyway, so that seems to solve the issue too.

Yeah, putting the check_cfs_rq_runtime inside of that condition was
specific to the exact pnt_fair code, and the specific places that
nr_running was and was not checked. pick_task_fair doesn't care about
any of that, and if instead put_prev manages to throttle the picked
task, we can still successfully switch to it for a moment.

Reviewed-by: Ben Segall <bsegall@google.com>

>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/sched/fair.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8435,11 +8435,11 @@ static struct task_struct *pick_task_fai
>  				update_curr(cfs_rq);
>  			else
>  				curr = NULL;
> -
> -			if (unlikely(check_cfs_rq_runtime(cfs_rq)))
> -				goto again;
>  		}
>  
> +		if (unlikely(check_cfs_rq_runtime(cfs_rq)))
> +			goto again;
> +
>  		se = pick_next_entity(cfs_rq);
>  		cfs_rq = group_cfs_rq(se);
>  	} while (cfs_rq);