linux-kernel - Re: [PATCH] sched/deadline: Skip overflow check if 0 capacity

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Message-ID: <Zy4RWJPH9jxew_7G@jlelli-thinkpadt14gen4.remote.csb>
Date: Fri, 8 Nov 2024 14:25:44 +0100
From: Juri Lelli <juri.lelli@...hat.com>
To: Waiman Long <longman@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org, Phil Auld <pauld@...hat.com>,
	Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH] sched/deadline: Skip overflow check if 0 capacity

On 07/11/24 23:29, Waiman Long wrote:
> By properly setting up a 1-cpu sched domain (partition) with no
> task, it was found that offlining that particular CPU failed because
> dl_bw_check_overflow() in cpuset_cpu_inactive() returned -EBUSY. This
> is due to the fact that dl_bw_capacity() return 0 as the sched domain
> has no active CPU causing a false positive in the overflow check.
> 
> Fix this corner case by skipping the __dl_overflow() check in
> dl_bw_manage() when the returned capacity is 0.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  kernel/sched/deadline.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index be1b917dc8ce..0195f350d6d3 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -3479,7 +3479,13 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw)
>  	} else {
>  		unsigned long cap = dl_bw_capacity(cpu);
>  
> -		overflow = __dl_overflow(dl_b, cap, 0, dl_bw);
> +		/*
> +		 * In the unlikely case of 0 capacity (e.g. a sched domain
> +		 * with no active CPUs), skip the overflow check as it will
> +		 * always return a false positive.
> +		 */
> +		if (likely(cap))
> +			overflow = __dl_overflow(dl_b, cap, 0, dl_bw);

The remaining total_bw that make this check fail should be the one
relative to the dl_server on the cpu that is going offline. Wonder if we
shouldn't rather clean that up (remove dl_server contribution) before we
get to this point during an hotplug operation. Need to think about it a
little more.

Thanks,
Juri