linux-kernel - Re: [PATCH] schedule: use unlikely()

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Date:   Fri, 24 Nov 2017 08:38:19 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] schedule: use unlikely()


* Mikulas Patocka <mpatocka@...hat.com> wrote:

> A small patch for schedule(), so that the code goes straght in the common
> case.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
> 
> ---
>  include/linux/blkdev.h |    2 +-
>  kernel/sched/core.c    |    2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6/include/linux/blkdev.h
> ===================================================================
> --- linux-2.6.orig/include/linux/blkdev.h
> +++ linux-2.6/include/linux/blkdev.h
> @@ -1308,7 +1308,7 @@ static inline bool blk_needs_flush_plug(
>  {
>  	struct blk_plug *plug = tsk->plug;
>  
> -	return plug &&
> +	return unlikely(plug != NULL) &&
>  		(!list_empty(&plug->list) ||
>  		 !list_empty(&plug->mq_list) ||
>  		 !list_empty(&plug->cb_list));

That's an unrelated change.

> Index: linux-2.6/kernel/sched/core.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched/core.c
> +++ linux-2.6/kernel/sched/core.c
> @@ -3405,7 +3405,7 @@ void __noreturn do_task_dead(void)
>  
>  static inline void sched_submit_work(struct task_struct *tsk)
>  {
> -	if (!tsk->state || tsk_is_pi_blocked(tsk))
> +	if (!tsk->state || unlikely(tsk_is_pi_blocked(tsk)))
>  		return;
>  	/*
>  	 * If we are going to sleep and we have plugged IO queued,

Do these changes actually change the generated assembly code?

Thanks,

	Ingo