linux-kernel - Re: [PATCH] sched: task_struct: Fill unconditional hole induced by sched

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Message-ID: <202110052146.D639C70@keescook>
Date:   Tue, 5 Oct 2021 21:48:51 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] sched: task_struct: Fill unconditional hole induced by
 sched_entity

On Thu, Sep 23, 2021 at 07:54:50PM -0700, Kees Cook wrote:
> With struct sched_entity before the other sched entities, its alignment
> won't induce a struct hole. This saves 64 bytes in defconfig task_struct:

Friendly ping. Can someone snag this for -tip please?

Thanks!

-Kees

> 
> Before:
> 	...
>         unsigned int               rt_priority;          /*   120     4 */
> 
>         /* XXX 4 bytes hole, try to pack */
> 
>         /* --- cacheline 2 boundary (128 bytes) --- */
>         const struct sched_class  * sched_class;         /*   128     8 */
> 
>         /* XXX 56 bytes hole, try to pack */
> 
>         /* --- cacheline 3 boundary (192 bytes) --- */
>         struct sched_entity        se __attribute__((__aligned__(64))); /*   192   448 */
>         /* --- cacheline 10 boundary (640 bytes) --- */
>         struct sched_rt_entity     rt;                   /*   640    48 */
>         struct sched_dl_entity     dl __attribute__((__aligned__(8))); /*   688   224 */
>         /* --- cacheline 14 boundary (896 bytes) was 16 bytes ago --- */
> 
> After:
> 	...
>         unsigned int               rt_priority;          /*   120     4 */
> 
>         /* XXX 4 bytes hole, try to pack */
> 
>         /* --- cacheline 2 boundary (128 bytes) --- */
>         struct sched_entity        se __attribute__((__aligned__(64))); /*   128   448 */
>         /* --- cacheline 9 boundary (576 bytes) --- */
>         struct sched_rt_entity     rt;                   /*   576    48 */
>         struct sched_dl_entity     dl __attribute__((__aligned__(8))); /*   624   224 */
>         /* --- cacheline 13 boundary (832 bytes) was 16 bytes ago --- */
> 
> Summary diff:
> -	/* size: 7040, cachelines: 110, members: 188 */
> +	/* size: 6976, cachelines: 109, members: 188 */
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  include/linux/sched.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 39039ce8ac4c..27ed1d40028f 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -775,10 +775,10 @@ struct task_struct {
>  	int				normal_prio;
>  	unsigned int			rt_priority;
>  
> -	const struct sched_class	*sched_class;
>  	struct sched_entity		se;
>  	struct sched_rt_entity		rt;
>  	struct sched_dl_entity		dl;
> +	const struct sched_class	*sched_class;
>  
>  #ifdef CONFIG_SCHED_CORE
>  	struct rb_node			core_node;
> -- 
> 2.30.2
> 

-- 
Kees Cook