linux-kernel - Re: [PATCH 1/3] wait: introduce wait_event_cmd

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Date:	Tue, 28 Apr 2015 16:13:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
Cc:	neilb@...e.de, linux-raid@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 1/3] wait: introduce wait_event_cmd_exclusive

On Mon, Apr 27, 2015 at 12:51:01PM +0800, Yuanhan Liu wrote:
> It's just a variant of wait_event_cmd, with exclusive flag being set.
> 
> For cases like RAID5, which puts many processes to sleep until 1/4
> resources are free, a wake_up wakes up all processes to run, but
> there is one process being able to get the resource as it's protected
> by a spin lock. That ends up introducing heavy lock contentions, and
> hurts performance badly.
> 
> Here introduce wait_event_cmd_exclusive to relieve the lock contention
> naturally by letting wake_up() just wake up one process.
> 
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Yuanhan Liu <yuanhan.liu@...ux.intel.com>
> ---
>  include/linux/wait.h | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/wait.h b/include/linux/wait.h
> index 2db8334..6c3b4de 100644
> --- a/include/linux/wait.h
> +++ b/include/linux/wait.h
> @@ -358,10 +358,18 @@ do {									\
>  	__ret;								\
>  })
>  
> -#define __wait_event_cmd(wq, condition, cmd1, cmd2)			\
> -	(void)___wait_event(wq, condition, TASK_UNINTERRUPTIBLE, 0, 0,	\
> +#define __wait_event_cmd(wq, condition, cmd1, cmd2, exclusive)		\
> +	(void)___wait_event(wq, condition, TASK_UNINTERRUPTIBLE, exclusive, 0, \
>  			    cmd1; schedule(); cmd2)
>  
> +
> +#define wait_event_cmd_exclusive(wq, condition, cmd1, cmd2)			\
> +do {									\
> +	if (condition)							\
> +		break;							\
> +	__wait_event_cmd(wq, condition, cmd1, cmd2, 1);			\
> +} while (0)
> +
>  /**
>   * wait_event_cmd - sleep until a condition gets true
>   * @wq: the waitqueue to wait on
> @@ -380,7 +388,7 @@ do {									\
>  do {									\
>  	if (condition)							\
>  		break;							\
> -	__wait_event_cmd(wq, condition, cmd1, cmd2);			\
> +	__wait_event_cmd(wq, condition, cmd1, cmd2, 0);			\
>  } while (0)
>  

No, that's wrong, its assumed that wait*() and __wait*() have the same
arguments.
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