linux-kernel - Re: [PATCH] sched: remove unused cpumask variable in mm_cid

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Message-ID: <yc3bcn76b6jcdcbnoegwi6bigccor32fuevg66o2lqdewem7dc@nvujb2gazknq>
Date: Tue, 21 Oct 2025 14:01:42 +0200
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: Peter Zijlstra <peterz@...radead.org>, 
	Thomas Gleixner <tglx@...utronix.de>
Cc: Vlastimil Babka <vbabka@...e.cz>, Breno Leitao <leitao@...ian.org>, 
	Kriish Sharma <kriish.sharma2006@...il.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>, linux-kernel@...r.kernel.org, david.hunter.linux@...il.com, 
	skhan@...uxfoundation.org, Menglong Dong <menglong8.dong@...il.com>, 
	André Draszik <andre.draszik@...aro.org>, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
	Aditya Gollamudi <adigollamudi@...il.com>, Kevin Brodsky <kevin.brodsky@....com>, 
	Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>, Dan Carpenter <error27@...il.com>
Subject: Re: [PATCH] sched: remove unused cpumask variable in mm_cid_get()

Hello Peter,

On Tue, Oct 14, 2025 at 12:34:39PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 14, 2025 at 12:13:23PM +0200, Vlastimil Babka wrote:
> > On 10/14/25 11:56, Breno Leitao wrote:
> > > On Thu, Oct 09, 2025 at 07:48:18PM +0000, Kriish Sharma wrote:
> > >> The variable 'cpumask' in mm_cid_get() was assigned but never used,
> > >> causing the following build error with -Werror:
> > >> 
> > >> kernel/sched/sched.h: In function ‘mm_cid_get’:
> > >> kernel/sched/sched.h:3743:25: error: variable ‘cpumask’ set but not used [-Werror=unused-but-set-variable]
> > >>  3743 |         struct cpumask *cpumask;
> > >>       |                         ^~~~~~~
> > > 
> > > Thanks for the fix. I am hitting the same issue in my builds.
> > 
> > Let me add why this years old small issue became much more problematic in
> > 6.18-rc1. When I want to test my own files I'm developing on with e.g. "make
> > W=1 mm/slub.o", the W=1 hits earlier in:
> > 
> >   CC      kernel/sched/rq-offsets.s
> > In file included from kernel/sched/rq-offsets.c:5:
> > kernel/sched/sched.h:3718:18: error: variable 'cpumask' set but not used
> > [-Werror,-Wunused-but-set-variable]
> >  3718 |         struct cpumask *cpumask;
> >       |                         ^
> > 1 error generated.
> > make[2]: *** [scripts/Makefile.build:182: kernel/sched/rq-offsets.s] Error 1
> > 
> > So I can't get to the part where I test-compile my own code with W=1. So
> > fixing this ASAP in 6.18 would be appreciated, thanks!
> > 
> > FWIW I've bisected this to commit
> > 378b7708194f ("sched: Make migrate_{en,dis}able() inline")

There are several other submissions of the same patch with different
commit logs; I found:

https://lore.kernel.org/all/20251002-sched-w1-v1-1-a6fdf549d179@linaro.org/
https://lore.kernel.org/all/20251009194818.1587650-1-kriish.sharma2006@gmail.com/
https://lore.kernel.org/all/20251015091935.2977229-1-andriy.shevchenko@linux.intel.com/
https://lore.kernel.org/all/20251020221728.177983-1-adigollamudi@gmail.com/
https://lore.kernel.org/all/20251017073050.2411988-1-kevin.brodsky@arm.com/ 

Also Krzysztof's build bot is very unhappy:
https://krzk.eu/#/builders/135

> People using W=1 and WERROR can keep the pieces. Anyway, this is a much
> more coherent explanation that the original patch.

Can we please get this fixed even though you don't bother about W=1
builds? There seem to be others who do. And note that even

	make W=1 drivers/pwm/

is broken due to that, so it affects also maintainers who only want W=1
for their own subtree.

Regarding the Fixes line: Vlastimil Babka bisected it to 378b7708194f
("sched: Make migrate_{en,dis}able() inline"), but I think this is just
the commit that made the compiler notice that. IMHO Andy identified the
more plausible commit with:

Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced by mm_cid")

.

Note there is a lkp report about André's patch (i.e. the first in my
list above) at
https://lore.kernel.org/all/202510041546.DvhFLp2x-lkp@intel.com/#t. I
don't understand the issue found there, but maybe someone should before
the patch is applied.

Best regards
Uwe

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