linux-kernel - Re: [PATCH] sched/rt: fix incorrect schedstats for rt thread

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Message-ID: <20260108111632.GH272712@noisy.programming.kicks-ass.net>
Date: Thu, 8 Jan 2026 12:16:32 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Dengjun Su <dengjun.su@...iatek.com>
Cc: linux-kernel@...r.kernel.org, mike.zhang@...iatek.com,
	haiqiang.gong@...iatek.com, peijun.huang@...iatek.com,
	Ingo Molnar <mingo@...hat.com>, 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>,
	Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH] sched/rt: fix incorrect schedstats for rt thread

On Thu, Jan 08, 2026 at 11:13:07AM +0800, Dengjun Su wrote:
> For RT thread, only 'set_next_task_rt' will call
> 'update_stats_wait_end_rt' to update schedstats information.
> However, during the RT migration process,
> 'update_stats_wait_start_rt' will be called twice, which
> will cause the values of wait_max and wait_sum to be incorrect.

Right, that looses time. Also note that I think dl has the same issue.

> The specific output as follows:
> $ cat /proc/6046/task/6046/sched | grep wait
> wait_start                                   :             0.000000
> wait_max                                     :        496717.080029
> wait_sum                                     :       7921540.776553
> 
> Add 'update_stats_wait_end_rt' in 'update_stats_dequeue_rt' to
> update schedstats information when dequeue_task.

This needs a few more words on why this is correct -- notably it took me
a little time to find the 'task_on_rq_migrating()' case in
__update_stats_wait_end() which makes this not actually 'end'.

But then the corresponding clause in __update_stats_wait_start() gives
me a headache:

 'wait_start > prev_wait_start'

I mean, wtf. Should that not equally be using task_on_rq_migrating() ?

Can you please take a hard look at all that and fix up things all-round?