linux-kernel - Re: [PATCH 2/2] mm: compaction: Minimise the time IRQs are disabled while isolating pages for migration

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Message-ID: <20110301161900.GA21860@random.random>
Date:	Tue, 1 Mar 2011 17:19:00 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arthur Marsh <arthur.marsh@...ernode.on.net>,
	Clemens Ladisch <cladisch@...glemail.com>,
	Linux-MM <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: compaction: Minimise the time IRQs are
 disabled while isolating pages for migration

On Wed, Mar 02, 2011 at 12:35:58AM +0900, Minchan Kim wrote:
> On Tue, Mar 01, 2011 at 01:49:25PM +0900, KAMEZAWA Hiroyuki wrote:
> > On Tue, 1 Mar 2011 13:11:46 +0900
> > Minchan Kim <minchan.kim@...il.com> wrote:
> > 
> > > On Tue, Mar 01, 2011 at 08:42:09AM +0900, KAMEZAWA Hiroyuki wrote:
> > > > On Mon, 28 Feb 2011 10:18:27 +0000
> > > > Mel Gorman <mel@....ul.ie> wrote:
> > > > 
> > > > > > BTW, can't we drop disable_irq() from all lru_lock related codes ?
> > > > > > 
> > > > > 
> > > > > I don't think so - at least not right now. Some LRU operations such as LRU
> > > > > pagevec draining are run from IPI which is running from an interrupt so
> > > > > minimally spin_lock_irq is necessary.
> > > > > 
> > > > 
> > > > pagevec draining is done by workqueue(schedule_on_each_cpu()). 
> > > > I think only racy case is just lru rotation after writeback.
> > > 
> > > put_page still need irq disable.
> > > 
> > 
> > Aha..ok. put_page() removes a page from LRU via __page_cache_release().
> > Then, we may need to remove a page from LRU under irq context.
> > Hmm...
> 
> But as __page_cache_release's comment said, normally vm doesn't release page in
> irq context. so it would be rare.
> If we can remove it, could we change all of spin_lock_irqsave with spin_lock?
> If it is right, I think it's very desirable to reduce irq latency.
> 
> How about this? It's totally a quick implementation and untested. 
> I just want to hear opinions of you guys if the work is valuable or not before
> going ahead.

pages freed from irq shouldn't be PageLRU.

deferring freeing to workqueue doesn't look ok. firewall loads runs
only from irq and this will cause some more work and a delay in the
freeing. I doubt it's worhwhile especially for the lru_lock.
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