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

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Message-ID: <c8dd612a-1851-471f-b5c5-f4201593e7b0@linux.ibm.com>
Date: Wed, 16 Oct 2024 15:06:54 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org
Cc: 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



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?


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);
>   }