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

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Date: Fri, 21 Jun 2024 15:13:19 -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 21, 2024 at 09:07:19PM +0200, Sebastian Andrzej Siewior wrote:
> On 2024-06-21 14:49:23 [-0400], Kent Overstreet wrote:
> > On Fri, Jun 21, 2024 at 08:29:15PM +0200, Sebastian Andrzej Siewior wrote:
> > > On 2024-06-21 10:20:58 [-0400], Kent Overstreet wrote:
> > > > On Fri, Jun 21, 2024 at 12:27:50PM +0200, Sebastian Andrzej Siewior 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.
> > > > 
> > > > We sure we want it at the end? we do want it on a hot cacheline
> > > 
> > > Well, the front is bad.
> > > Looking at pgalloc_tag_add() and its callers, there is no task_struct
> > > touching. alloc_tag_save()/restore might be the critical one. This is
> > > 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.