linux-kernel - Re: [PATCH v3] sched: reorder some fields in struct rq

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Message-ID: <CAP_z_CiLDymFxijLK2Wv5UsydbOx1SoM6V6hYCgFckLs6HtBFQ@mail.gmail.com>
Date: Wed, 29 Oct 2025 11:56:56 -0700
From: Blake Jones <blakejones@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Madadi Vineeth Reddy <vineethr@...ux.ibm.com>, 
	Josh Don <joshdon@...gle.com>, Andrew Morton <akpm@...ux-foundation.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
Subject: Re: [PATCH v3] sched: reorder some fields in struct rq

On Wed, Oct 29, 2025 at 2:14 AM Peter Zijlstra <peterz@...radead.org> wrote:
> Most places that take rq->lock also update rq->clock. So I was wondering
> if it made sense to put them together. I don't have numbers either.

Makes sense. I think I'll leave it out of this patch since I don't
have numbers for it, but it's an compelling enough idea that I might
try to get some numbers and come back with a followup.

> > The line that I've moved it to has your queue_mask, so that's probably not
> > the best place. How about if I move it to the end of the structure, next to
> > cfsb_csd and cfsb_csd_list, since those don't seem to get used very often?
>
> Works for me.

Great.

> > Well, this is by far one of the hottest kernel data structures we see at
> > Google, and it's not an obvious thing for a security vuln to target. So I'd
> > be quite happy to add __no_randomize_layout; if you agree, I'll add it.
>
> Lets make it so.

Done.

I've made these changes and fleshed out the comment at the top of the
structure a bit more, and I sent the result out as v4:
https://lore.kernel.org/lkml/20251029185458.3040228-1-blakejones@google.com/T/#u

Thanks.

Blake