linux-kernel - Re: [patch 2/2] sched/debug: Remove need

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Message-ID: <85f483ed-845c-a25f-3558-e1a8629e9200@google.com>
Date: Tue, 7 Jan 2025 12:15:19 -0800 (PST)
From: David Rientjes <rientjes@...gle.com>
To: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>, 
    Josh Don <joshdon@...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
Subject: Re: [patch 2/2] sched/debug: Remove need_resched ratelimiting for
 warnings

On Tue, 7 Jan 2025, Madadi Vineeth Reddy wrote:

> Hi David Rientjes,
> 
> On 07/01/25 02:09, David Rientjes wrote:
> > The need_resched warnings are controlled by two tunables in debugfs:
> >  - latency_warn_ms
> >  - latency_warn_once
> > 
> > By default, latency_warn_once is enabled.  Thus, a need_resched warning
> > is only emitted once per boot.
> > 
> > If the user configures this to not be the case and changes the default,
> > then allow the user to also control the threshold through latency_warn_ms
> > that these warnings trigger.  Do not impose our own ratelimiting on top
> > that may make it appear like there are no cases where need_resched is set
> > for longer than the threshold.
> 
> Any idea why it was initially kept to one warning per hour?
> 

Adding Josh Don who may have insight into this historically.

> The possible reasons that come to mind are to prevent excessive logging under
> high CPU contention, as well as to ensure that a warning logged once an hour
> indicates the issue is not caused by a short workload spike. Additionally,
> this rate limit might help avoid impacting system performance due to excessive
> logging.
> 
> However, if the default value of latency_warn_once is changed to disable it, it
> may be acceptable to bypass the rate limit, as it would indicate a preference
> for logging over performance.
> 

Right, I think this should be entirely up to what the admin configures in 
debugfs.  If they elect to disable latency_warn_once, we'll simply emit 
the information as often as they specify in latency_warn_ms and not add 
our own ratelimiting on top.  If they have a preference for lots of 
logging, so be it, let's not hide that data.

> Thoughts?
> 
> Thanks,
> Madadi Vineeth Reddy
> 
> > 
> > Signed-off-by: David Rientjes <rientjes@...gle.com>
> > ---
> >  kernel/sched/debug.c | 5 -----
> >  1 file changed, 5 deletions(-)
> > 
> > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> > --- a/kernel/sched/debug.c
> > +++ b/kernel/sched/debug.c
> > @@ -1293,11 +1293,6 @@ void proc_sched_set_task(struct task_struct *p)
> >  
> >  void resched_latency_warn(int cpu, u64 latency)
> >  {
> > -	static DEFINE_RATELIMIT_STATE(latency_check_ratelimit, 60 * 60 * HZ, 1);
> > -
> > -	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);
> >  	dump_stack();
> 
>