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

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Message-ID: <20240628094944.QwcHkf8J@linutronix.de>
Date: Fri, 28 Jun 2024 11:49:44 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc: 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>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	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 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