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

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Message-ID: <w2qopormdwuh54vropw6sgvadnivjnrgvmpurud2cu6nannfj3@xxrs2r6qt7zi>
Date: Fri, 28 Jun 2024 16:20:27 -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 Fri, Jun 28, 2024 at 09:55:53PM +0200, Sebastian Andrzej Siewior wrote:
> On 2024-06-28 15:35:38 [-0400], Kent Overstreet wrote:
> > On Fri, Jun 28, 2024 at 11:49:44AM +0200, Sebastian Andrzej Siewior wrote:
> > > On 2024-06-21 12:27:52 [+0200], To linux-mm@...ck.org wrote:
> > > > The alloc_tag member has been added to task_struct at the very
> > > > beginning. This is a pointer and on 64bit architectures it forces 4 byte
> > > > padding after `ptrace' and then forcing another another 4 byte padding
> > > > after `on_cpu'. A few members later, `se' requires a cacheline aligned
> > > > due to struct sched_avg resulting in 52 hole before `se'.
> > > > 
> > > > This is the case on 64bit-SMP architectures.
> > > > The 52 byte hole can be avoided by moving alloc_tag away where it
> > > > currently resides.
> > > > 
> > > > Move alloc_tag to the end of task_struct. There is likely a hole before
> > > > `thread' due to its alignment requirement and the previous members are
> > > > likely to be already pointer-aligned.
> > > > 
> > > > Fixes: 22d407b164ff7 ("lib: add allocation tagging support for memory allocation profiling")
> > > > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > > 
> > > Could we please get this merged and worry about possible performance
> > > regression later? Or once there is a test case or an idea where this
> > > pointer might fit better but clearly the current situation is worse.
> > 
> > Sebastian, I gave you review feedback on your patch; if you can
> > incorporate it into a new version your patch will sail right in.
> 
> 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.