linux-kernel - Re: [PATCH] sched/task_struct: Move alloc

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Date: Sun, 30 Jun 2024 17:23:36 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Andrew Morton <akpm@...ux-foundation.org>, Ben Segall <bsegall@...gle.com>, 
	Daniel Bristot de Oliveira <bristot@...hat.com>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, 
	Klara Modin <klarasmodin@...il.com>, Mel Gorman <mgorman@...e.de>, 
	Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Suren Baghdasaryan <surenb@...gle.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Valentin Schneider <vschneid@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] sched/task_struct: Move alloc_tag to the end of the
 struct.

On Sun, Jun 30, 2024 at 11:11:42PM GMT, Sebastian Andrzej Siewior wrote:
> On 2024-06-28 16:20:27 [-0400], Kent Overstreet wrote:
> > > Kent, you said you didn't want it where it currently is. Fine. You said
> > > you want it at the front next to `flags'. This isn't going to work since
> > > there is no space left. You didn't make another suggestion or say how to
> > > make room.
> > 
> > It doesn't need to be on the exact same cacheline, just as near as you
> > can get it.
> 
> the first possible thing would be somewhere after the scheduler.
> However, what difference does it make if it s two cache lines later or
> more?  I don't understand the requirement "closer".

take advantage of CPU prefetching; CPUs will bring in more than just the
cacheline you touched because 64 bytes is small and it's cheap to fetch
from the same DRAM bank while it's open.