linux-kernel - Re: [PATCH 1/2] sched/wait: add round robin wakeup mode

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Date:	Tue, 10 Feb 2015 10:03:36 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Baron <jbaron@...mai.com>
Cc:	mingo@...hat.com, viro@...iv.linux.org.uk,
	akpm@...ux-foundation.org, normalperson@...t.net,
	davidel@...ilserver.org, mtk.manpages@...il.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/wait: add round robin wakeup mode

On Mon, Feb 09, 2015 at 11:06:17PM -0500, Jason Baron wrote:
> On 02/09/2015 04:50 PM, Peter Zijlstra wrote:
> > On Mon, Feb 09, 2015 at 08:05:57PM +0000, Jason Baron wrote:
> >> diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
> >> index 852143a..17d1039 100644
> >> --- a/kernel/sched/wait.c
> >> +++ b/kernel/sched/wait.c
> >> @@ -71,8 +71,11 @@ static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
> >>  		unsigned flags = curr->flags;
> >>  
> >>  		if (curr->func(curr, mode, wake_flags, key) &&
> >> -				(flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
> >> +			       (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive) {
> >> +			if (flags & WQ_FLAG_ROUND_ROBIN)
> >> +				list_move_tail(&curr->task_list, &q->task_list);
> >>  			break;
> >> +		}
> >>  	}
> >>  }
> > I think you meant to write something like:
> >
> > 	if (curr->func(curr, mode, wake_flags, key) &&
> > 	    (flags & WQ_FLAG_EXCLUSIVE)) {
> > 		if (flag & WQ_FLAG_ROUND_ROBIN)
> > 			list_move_tail(&curr->task_list, &q->task_list);
> > 		if (!--nr_exclusive)
> > 			break;
> > 	}
> >
> > Otherwise can only work for nr_exclusive==1.
> 
> Indeed. I'm also wondering if its worth avoiding the list_move_tail()
> for the case where nr_exclusive is initially 0. IE the wake all case,
> where we are just going to end up doing a bunch of list_move_tail()
> calls, but end up in the same state.

After writing this email, it occurred to me that you could probably do
this with a custom wake function.

Where autoremove_wake_function() does a list_del_init() you could do a
rotate_wake_function() that does list_move_tail().

That would avoid the entire WQ flag muckery.
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