linux-kernel - Re: [PATCH v2 6/6] sched: warn for high latency with TIF_NEED_RESCHED

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Message-ID: <20241114091614.jSmSI78r@linutronix.de>
Date: Thu, 14 Nov 2024 10:16:14 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, peterz@...radead.org, tglx@...utronix.de,
	paulmck@...nel.org, mingo@...nel.org, 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,
	sshegde@...ux.ibm.com, boris.ostrovsky@...cle.com,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 6/6] sched: warn for high latency with
 TIF_NEED_RESCHED_LAZY

On 2024-11-06 12:17:58 [-0800], Ankur Arora wrote:
> Add support for warning if the TIF_NEED_RESCHED_LAZY bit is set
> without rescheduling for more than the latency_warn_ms period.

You fail to explain _why_ it is required to also check
TIF_NEED_RESCHED_LAZY to not be set.

The problem with NEED_RESCHED set but no scheduling in 100ms is a long
preempt-off or IRQ-off region .
The problem with NEED_RESCHED_LAZY set but no scheduling in 100ms is a
missing timer tick in that time. Also the previous mentioned things
might have happen.
And the code acting on NEED_RESCHED_LAZY is invoked before this check
is.

Sebastian