linux-kernel - Re: [PATCH 1/7] sched: warn for high latency with TIF_NEED_RESCHED

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Message-ID: <87iktli7o6.fsf@oracle.com>
Date: Mon, 21 Oct 2024 12:21:13 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        peterz@...radead.org, tglx@...utronix.de, paulmck@...nel.org,
        mingo@...nel.org, bigeasy@...utronix.de, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        vschneid@...hat.com, frederic@...nel.org, efault@....de
Subject: Re: [PATCH 1/7] sched: warn for high latency with
 TIF_NEED_RESCHED_LAZY


Shrikanth Hegde <sshegde@...ux.ibm.com> writes:

> On 10/9/24 22:24, Ankur Arora wrote:
>> resched_latency_warn() now also warns if TIF_NEED_RESCHED_LAZY is set
>> without rescheduling for more than the latency_warn_ms period.
>>
>
> I am bit confused here. Why do we need to warn if LAZY is set for a long time?
>
> If lazy set, the subsequent tick, it would be set to upgraded to NEED_RESCHED.
>
> Since the value of latency_warn_ms=100ms, that means even on system with HZ=100,
> that means 10 ticks before that warning would be printed no?

That's a fair point. However, the assumption there is that there are no
bugs in upgrade on tick or that there's no situation in which the tick
is off for a prolonged period.

Ankur

> IIUC, the changelog c006fac556e40 ("sched: Warn on long periods of pending
> need_resched") has the concern of need_resched set but if it is non-preemptible
> kernel it would spend a lot of time in kernel mode. In that case print a
> warning.
>
> If someone enables Lazy, that means it is preemptible and probably this whole
> notion of resched_latency_warn doesn't apply to lazy. Please correct me if i am
> not understanding this correctly.
>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Peter Ziljstra <peterz@...radead.org>
>> Cc: Juri Lelli <juri.lelli@...hat.com>
>> Cc: Vincent Guittot <vincent.guittot@...aro.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>> ---
>>   kernel/sched/core.c  | 2 +-
>>   kernel/sched/debug.c | 7 +++++--
>>   2 files changed, 6 insertions(+), 3 deletions(-)
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 694bfcf153cb..1229766b704e 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -5571,7 +5571,7 @@ static u64 cpu_resched_latency(struct rq *rq)
>>   	if (sysctl_resched_latency_warn_once && warned_once)
>>   		return 0;
>>   -	if (!need_resched() || !latency_warn_ms)
>> +	if ((!need_resched() && !tif_need_resched_lazy()) || !latency_warn_ms)
>>   		return 0;
>>     	if (system_state == SYSTEM_BOOTING)
>> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
>> index 9abcc6ead11b..f0d551ba64bb 100644
>> --- a/kernel/sched/debug.c
>> +++ b/kernel/sched/debug.c
>> @@ -1293,9 +1293,12 @@ 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);
>> +	char *nr;
>> +
>> +	nr = tif_need_resched() ? "need_resched" : "need_resched_lazy";
>>     	WARN(__ratelimit(&latency_check_ratelimit),
>> -	     "sched: CPU %d need_resched set for > %llu ns (%d ticks) "
>> +	     "sched: CPU %d %s set for > %llu ns (%d ticks) "
>>   	     "without schedule\n",
>> -	     cpu, latency, cpu_rq(cpu)->ticks_without_resched);
>> +	     cpu, nr, latency, cpu_rq(cpu)->ticks_without_resched);
>>   }


--
ankur