linux-kernel - Re: [PATCH] mm: filemap: Avoid unnecessary barriers and waitqueue lookups in unlock

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Message-ID: <20140522150451.GX30445@twins.programming.kicks-ass.net>
Date:	Thu, 22 May 2014 17:04:51 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] mm: filemap: Avoid unnecessary barriers and waitqueue
 lookups in unlock_page fastpath v7

On Thu, May 22, 2014 at 03:40:45PM +0100, Mel Gorman wrote:

> > +static bool __wake_up_common(wait_queue_head_t *q, unsigned int mode,
> >  			int nr_exclusive, int wake_flags, void *key)
> >  {
> >  	wait_queue_t *curr, *next;
> > +	bool woke = false;
> >  
> >  	list_for_each_entry_safe(curr, next, &q->task_list, task_list) {
> >  		unsigned flags = curr->flags;
> >  
> > +		if (curr->func(curr, mode, wake_flags, key)) {
> > +			woke = true;
> > +			if ((flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
> > +				break;
> > +		}
> >  	}
> > +
> > +	return woke;
> 
> Ok, thinking about this more I'm less sure.
> 
> There are cases where the curr->func returns false even though there is a
> task that needs to run -- task was already running or preparing to run. We
> potentially end up clearing PG_waiters while there are still tasks on the
> waitqueue. As __finish_wait checks if the waitqueue is empty and the last
> waiter clears the bit I think there is nothing to gain by trying to do the
> same job in __wake_up_page_bit.

Hmm, I think you're right, we need the test result from
wake_bit_function(), unpolluted by the ttwu return value.

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