From: Mel Gorman Subject: Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep Date: Mon, 16 May 2011 09:45:58 +0100 Message-ID: <20110516084558.GE5279@suse.de> References: <1305295404-12129-1-git-send-email-mgorman@suse.de> <1305295404-12129-5-git-send-email-mgorman@suse.de> <4DCFAA80.7040109@jp.fujitsu.com> <1305519711.4806.7.camel@mulgrave.site> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: James Bottomley , KOSAKI Motohiro , akpm@linux-foundation.org, colin.king@canonical.com, raghu.prabhu13@gmail.com, jack@suse.cz, chris.mason@oracle.com, cl@linux.com, penberg@kernel.org, riel@redhat.com, hannes@cmpxchg.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-ext4@vger.kernel.org To: Minchan Kim Return-path: Received: from cantor2.suse.de ([195.135.220.15]:50619 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751993Ab1EPIqC (ORCPT ); Mon, 16 May 2011 04:46:02 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, May 16, 2011 at 02:04:00PM +0900, Minchan Kim wrote: > On Mon, May 16, 2011 at 1:21 PM, James Bottomley > wrote: > > On Sun, 2011-05-15 at 19:27 +0900, KOSAKI Motohiro wrote: > >> (2011/05/13 23:03), Mel Gorman wrote: > >> > Under constant allocation pressure, kswapd can be in the situati= on where > >> > sleeping_prematurely() will always return true even if kswapd ha= s been > >> > running a long time. Check if kswapd needs to be scheduled. > >> > > >> > Signed-off-by: Mel Gorman > >> > --- > >> > =A0 mm/vmscan.c | =A0 =A04 ++++ > >> > =A0 1 files changed, 4 insertions(+), 0 deletions(-) > >> > > >> > diff --git a/mm/vmscan.c b/mm/vmscan.c > >> > index af24d1e..4d24828 100644 > >> > --- a/mm/vmscan.c > >> > +++ b/mm/vmscan.c > >> > @@ -2251,6 +2251,10 @@ static bool sleeping_prematurely(pg_data_= t *pgdat, int order, long remaining, > >> > =A0 =A0 unsigned long balanced =3D 0; > >> > =A0 =A0 bool all_zones_ok =3D true; > >> > > >> > + =A0 /* If kswapd has been running too long, just sleep */ > >> > + =A0 if (need_resched()) > >> > + =A0 =A0 =A0 =A0 =A0 return false; > >> > + > >> > >> Hmm... I don't like this patch so much. because this code does > >> > >> - don't sleep if kswapd got context switch at shrink_inactive_list > > > > This isn't entirely true: =A0need_resched() will be false, so we'll= follow > > the normal path for determining whether to sleep or not, in effect > > leaving the current behaviour unchanged. > > > >> - sleep if kswapd didn't > > > > This also isn't entirely true: whether need_resched() is true at th= is > > point depends on a whole lot more that whether we did a context swi= tch > > in shrink_inactive. It mostly depends on how long we've been runnin= g > > without giving up the CPU. =A0Generally that will mean we've been r= ound > > the shrinker loop hundreds to thousands of times without sleeping. > > > >> It seems to be semi random behavior. > > > > Well, we have to do something. =A0Chris Mason first suspected the h= ang was > > a kswapd rescheduling problem a while ago. =A0We tried putting > > cond_rescheds() in several places in the vmscan code, but to no ava= il. >=20 > Is it a result of test with patch of Hannes(ie, !pgdat_balanced)? >=20 > If it isn't, it would be nop regardless of putting cond_reshed at vms= can.c. > Because, although we complete zone balancing, kswapd doesn't sleep as > pgdat_balance returns wrong result. And at last VM calls > balance_pgdat. In this case, balance_pgdat returns without any work a= s > kswap couldn't find zones which have not enough free pages and goto > out. kswapd could repeat this work infinitely. So you don't have a > chance to call cond_resched. >=20 > But if your test was with Hanne's patch, I am very curious how come > kswapd consumes CPU a lot. >=20 > > The need_resched() in sleeping_prematurely() seems to be about the = best > > option. =A0The other option might be just to put a cond_resched() i= n > > kswapd_try_to_sleep(), but that will really have about the same eff= ect. >=20 > I don't oppose it but before that, I think we have to know why kswapd > consumes CPU a lot although we applied Hannes' patch. >=20 Because it's still possible for processes to allocate pages at the same rate kswapd is freeing them leading to a situation where kswapd does no= t consider the zone balanced for prolonged periods of time. --=20 Mel Gorman SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html