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Date: Thu, 22 May 2014 10:47:22 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Mel Gorman <mgorman@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>, Oleg Nesterov <oleg@redhat.com>,
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        David Howells <dhowells@redhat.com>
Subject: Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue
 lookups in unlock_page fastpath v5
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On Thu, 22 May 2014 09:46:43 +0100 Mel Gorman <mgorman@suse.de> wrote:

> > > If I'm still on track here, what happens if we switch to wake-all so we
> > > can avoid the dangling flag?  I doubt if there are many collisions on
> > > that hash table?
> > 
> > Wake-all will be ugly and loose a herd of waiters, all racing to
> > acquire, all but one of whoem will loose the race. It also looses the
> > fairness, its currently a FIFO queue. Wake-all will allow starvation.
> > 
> 
> And the cost of the thundering herd of waiters may offset any benefit of
> reducing the number of calls to page_waitqueue and waker functions.

Well, none of this has been demonstrated.

As I speculated earlier, hash chain collisions will probably be rare,
except for the case where a bunch of processes are waiting on the same
page.  And in this case, perhaps wake-all is the desired behavior.

Take a look at do_read_cache_page().  It does lock_page(), but it
doesn't actually *need* to.  It checks ->mapping and PG_uptodate and
then...  unlocks the page!  We could have used wait_on_page_locked()
there and permitted concurrent threads to run concurrently.

btw, I'm struggling a bit to understand why we bother checking
->mapping there as we're about to unlock the page anyway...