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

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Date: Fri, 21 Jun 2024 12:27:50 +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: [PATCH] sched/task_struct: Move alloc_tag to the end of the struct.

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>
---
 include/linux/sched.h | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 61591ac6eab6d..d76c61510ef1d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -770,10 +770,6 @@ struct task_struct {
 	unsigned int			flags;
 	unsigned int			ptrace;
 
-#ifdef CONFIG_MEM_ALLOC_PROFILING
-	struct alloc_tag		*alloc_tag;
-#endif
-
 #ifdef CONFIG_SMP
 	int				on_cpu;
 	struct __call_single_node	wake_entry;
@@ -1553,6 +1549,9 @@ struct task_struct {
 #ifdef CONFIG_USER_EVENTS
 	struct user_event_mm		*user_event_mm;
 #endif
+#ifdef CONFIG_MEM_ALLOC_PROFILING
+	struct alloc_tag		*alloc_tag;
+#endif
 
 	/*
 	 * New fields for task_struct should be added above here, so that
-- 
2.45.2