linux-kernel - Re: [patch 1/2] sched/debug: Change need_resched warnings to pr

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Message-ID: <833e94a8-738e-4107-81a9-68314cc99954@linux.ibm.com>
Date: Wed, 8 Jan 2025 00:48:23 +0530
From: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
To: David Rientjes <rientjes@...gle.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        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,
        Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Subject: Re: [patch 1/2] sched/debug: Change need_resched warnings to pr_err

Hi David Rientjes,

On 07/01/25 02:09, David Rientjes wrote:
> need_resched warnings, if enabled, are treated as WARNINGs.  If
> kernel.panic_on_warn is enabled, then this causes a kernel panic.
> 
> It's highly unlikely that a panic is desired for these warnings, only a
> stack trace is normally required to debug and resolve.
> 
> Thus, switch need_resched warnings to simply be a printk with an
> associated stack trace so they are no longer in scope for panic_on_warn.
> 
> Signed-off-by: David Rientjes <rientjes@...gle.com>
> ---
>  kernel/sched/debug.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -1295,8 +1295,10 @@ void resched_latency_warn(int cpu, u64 latency)
>  {
>  	static DEFINE_RATELIMIT_STATE(latency_check_ratelimit, 60 * 60 * HZ, 1);
>  
> -	WARN(__ratelimit(&latency_check_ratelimit),
> -	     "sched: CPU %d need_resched set for > %llu ns (%d ticks) "
> -	     "without schedule\n",
> -	     cpu, latency, cpu_rq(cpu)->ticks_without_resched);
> +	if (likely(!__ratelimit(&latency_check_ratelimit)))
> +		return;
> +
> +	pr_err("sched: CPU %d need_resched set for > %llu ns (%d ticks) without schedule\n",
> +	       cpu, latency, cpu_rq(cpu)->ticks_without_resched);

LGTM. While this is an issue, it doesn't necessarily indicate a critical failure that would
require the kernel to panic.

Nit: Would using pr_warn instead be too lenient in this case?

Reviewed-by: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>

Thanks,
Madadi Vineeth Reddy

> +	dump_stack();
>  }