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@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Yuanhan Liu <yuanhan.liu@xxxxxxxxxxxxxxx>
> ---
> 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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