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

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Message-ID: <20240621192252.-whn_Ae8@linutronix.de>
Date: Fri, 21 Jun 2024 21:22:52 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Kent Overstreet <kent.overstreet@...ux.dev>
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 2024-06-21 15:13:19 [-0400], Kent Overstreet wrote:
> > > > random code… Puh. So if the end is too cold, what about around the mm
> > > > pointer?
> > > 
> > > Not there, that's not actually that hot. It needs to be by
> > > task_struct->flags; that's used in the same paths.
> > 
> > But there is no space without the additional 52 bytes. Was this by any
> > chance on purpose?
> 
> No, that wasn't, and it doesn't have to be the exact same cacheline, but
> we do want it near current->flags; we store PF_MEMALLOC flags there that
> are converted to gfp flags and used exactly where we're using
> current->alloc_tag.

Hmm. `stack' and `usage' are the only two member that you would have to
move (away) in order the stash the conditional variable there. The
`ptrace' one uses the same flags as `flags' so it wouldn't make sense to
move that one.

Sebastian