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 11:27:53 +0100 Message-ID: <20110516102753.GF5279@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> <20110516084558.GE5279@suse.de> 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]:54397 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754382Ab1EPK2B (ORCPT ); Mon, 16 May 2011 06:28:01 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, May 16, 2011 at 05:58:59PM +0900, Minchan Kim wrote: > On Mon, May 16, 2011 at 5:45 PM, Mel Gorman wrote: > > 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 situ= ation where > >> >> > sleeping_prematurely() will always return true even if kswapd= has 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_da= ta_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_l= ist > >> > > >> > 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 effe= ct > >> > leaving the current behaviour unchanged. > >> > > >> >> - sleep if kswapd didn't > >> > > >> > This also isn't entirely true: whether need_resched() is true at= this > >> > point depends on a whole lot more that whether we did a context = switch > >> > in shrink_inactive. It mostly depends on how long we've been run= ning > >> > without giving up the CPU. =A0Generally that will mean we've bee= n round > >> > the shrinker loop hundreds to thousands of times without sleepin= g. > >> > > >> >> It seems to be semi random behavior. > >> > > >> > Well, we have to do something. =A0Chris Mason first suspected th= e hang was > >> > a kswapd rescheduling problem a while ago. =A0We tried putting > >> > cond_rescheds() in several places in the vmscan code, but to no = avail. > >> > >> Is it a result of =A0test with patch of Hannes(ie, !pgdat_balanced= )? > >> > >> If it isn't, it would be nop regardless of putting cond_reshed at = vmscan.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 wor= k as > >> kswap couldn't find zones which have not enough free pages and got= o > >> out. kswapd could repeat this work infinitely. So you don't have a > >> chance to call cond_resched. > >> > >> But if your test was with Hanne's patch, I am very curious how com= e > >> kswapd consumes CPU a lot. > >> > >> > The need_resched() in sleeping_prematurely() seems to be about t= he best > >> > option. =A0The other option might be just to put a cond_resched(= ) in > >> > kswapd_try_to_sleep(), but that will really have about the same = effect. > >> > >> I don't oppose it but before that, I think we have to know why ksw= apd > >> consumes CPU a lot although we applied Hannes' patch. > >> > > > > Because it's still possible for processes to allocate pages at the = same > > rate kswapd is freeing them leading to a situation where kswapd doe= s not > > consider the zone balanced for prolonged periods of time. >=20 > We have cond_resched in shrink_page_list, shrink_slab and balance_pgd= at. > So I think kswapd can be scheduled out although it's scheduled in > after a short time as task scheduled also need page reclaim. Although > all task in system need reclaim, kswapd cpu 99% consumption is a > natural result, I think. > Do I miss something? >=20 Lets see; shrink_page_list() only applies if inactive pages were isolated which in turn may not happen if all_unreclaimable is set in shrink_zones(). If for whatver reason, all_unreclaimable is set on all zones, we can miss calling cond_resched(). shrink_slab only applies if we are reclaiming slab pages. If the first shrinker returns -1, we do not call cond_resched(). If that first shrinker is dcache and __GFP_FS is not set, direct reclaimers will not shrink at all. However, if there are enough of them running or if one of the other shrinkers is running for a very long time, kswapd could be starved acquiring the shrinker_rwsem and never reaching the cond_resched(). balance_pgdat() only calls cond_resched if the zones are not balanced. For a high-order allocation that is balanced, it checks order-0 again. During that window, order-0 might have become unbalanced so it loops again for order-0 and returns that was reclaiming for order-0 to kswapd(). It can then find that a caller has rewoken kswapd for a high-order and re-enters balance_pgdat() without ever have called cond_resched(). While it appears unlikely, there are bad conditions which can result in cond_resched() being avoided. --=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