linux-kernel - Re: [PATCH 1/3 v2] wait: introduce wait_event_exclusive

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Date:	Thu, 30 Apr 2015 08:17:13 +1000
From:	NeilBrown <neilb@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>,
	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 1/3 v2] wait: introduce wait_event_exclusive_cmd

On Wed, 29 Apr 2015 11:31:09 +0200 Peter Zijlstra <peterz@...radead.org>
wrote:

> On Wed, Apr 29, 2015 at 10:48:53AM +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_exclusive_cmd 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>
> 
> Thanks!
> 
> Assuming you want to route this through Neil's MD tree:
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Thanks a lot Peter!

I've queued the three patches in my md tree for 4.2.

Thanks yliu!

NeilBrown


> 
> > ---
> >  include/linux/wait.h | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> > 
> > diff --git a/include/linux/wait.h b/include/linux/wait.h
> > index 2db8334..db78c72 100644
> > --- a/include/linux/wait.h
> > +++ b/include/linux/wait.h
> > @@ -358,6 +358,19 @@ do {									\
> >  	__ret;								\
> >  })
> >  
> > +#define __wait_event_exclusive_cmd(wq, condition, cmd1, cmd2)		\
> > +	(void)___wait_event(wq, condition, TASK_UNINTERRUPTIBLE, 1, 0,	\
> > +			    cmd1; schedule(); cmd2)
> > +/*
> > + * Just like wait_event_cmd(), except it sets exclusive flag
> > + */
> > +#define wait_event_exclusive_cmd(wq, condition, cmd1, cmd2)		\
> > +do {									\
> > +	if (condition)							\
> > +		break;							\
> > +	__wait_event_exclusive_cmd(wq, condition, cmd1, cmd2);		\
> > +} while (0)
> > +
> >  #define __wait_event_cmd(wq, condition, cmd1, cmd2)			\
> >  	(void)___wait_event(wq, condition, TASK_UNINTERRUPTIBLE, 0, 0,	\
> >  			    cmd1; schedule(); cmd2)
> > -- 
> > 1.9.0
> > 
> --
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