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Date: Fri, 28 Jun 2024 12:20:29 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Ben Segall
 <bsegall@google.com>, Daniel Bristot de Oliveira <bristot@redhat.com>,
 Dietmar Eggemann <dietmar.eggemann@arm.com>, Ingo Molnar
 <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>, Kent Overstreet
 <kent.overstreet@linux.dev>, Klara Modin <klarasmodin@gmail.com>, Mel
 Gorman <mgorman@suse.de>, Peter Zijlstra <peterz@infradead.org>, Steven
 Rostedt <rostedt@goodmis.org>, Suren Baghdasaryan <surenb@google.com>,
 Thomas Gleixner <tglx@linutronix.de>, Valentin Schneider
 <vschneid@redhat.com>, Vincent Guittot <vincent.guittot@linaro.org>
Subject: Re: [PATCH] sched/task_struct: Move alloc_tag to the end of the
 struct.
Message-Id: <20240628122029.ed510073c534b739ee385521@linux-foundation.org>
In-Reply-To: <20240628094944.QwcHkf8J@linutronix.de>
References: <20240621102750.oH9p1FQq@linutronix.de>
	<20240628094944.QwcHkf8J@linutronix.de>
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On Fri, 28 Jun 2024 11:49:44 +0200 Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> On 2024-06-21 12:27:52 [+0200], To linux-mm@kvack.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@linutronix.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.
> 

All in favor of saving 56 bytes from the task_struct, but we can do
that by moving various things around.  Was alloc_tag the best choice?