From: Minchan Kim Subject: Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep Date: Mon, 16 May 2011 17:58:59 +0900 Message-ID: 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=UTF-8 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: Mel Gorman Return-path: Received: from mail-qy0-f174.google.com ([209.85.216.174]:60636 "EHLO mail-qy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753255Ab1EPI7A convert rfc822-to-8bit (ORCPT ); Mon, 16 May 2011 04:59:00 -0400 In-Reply-To: <20110516084558.GE5279@suse.de> Sender: linux-ext4-owner@vger.kernel.org List-ID: 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 situat= ion where >> >> > sleeping_prematurely() will always return true even if kswapd h= as been >> >> > running a long time. Check if kswapd needs to be scheduled. >> >> > >> >> > Signed-off-by: Mel Gorman >> >> > --- >> >> > =C2=A0 mm/vmscan.c | =C2=A0 =C2=A04 ++++ >> >> > =C2=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, >> >> > =C2=A0 =C2=A0 unsigned long balanced =3D 0; >> >> > =C2=A0 =C2=A0 bool all_zones_ok =3D true; >> >> > >> >> > + =C2=A0 /* If kswapd has been running too long, just sleep */ >> >> > + =C2=A0 if (need_resched()) >> >> > + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=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_lis= t >> > >> > This isn't entirely true: =C2=A0need_resched() will be false, so w= e'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 t= his >> > point depends on a whole lot more that whether we did a context sw= itch >> > in shrink_inactive. It mostly depends on how long we've been runni= ng >> > without giving up the CPU. =C2=A0Generally that will mean we've be= en round >> > the shrinker loop hundreds to thousands of times without sleeping. >> > >> >> It seems to be semi random behavior. >> > >> > Well, we have to do something. =C2=A0Chris Mason first suspected t= he hang was >> > a kswapd rescheduling problem a while ago. =C2=A0We tried putting >> > cond_rescheds() in several places in the vmscan code, but to no av= ail. >> >> Is it a result of =C2=A0test with patch of Hannes(ie, !pgdat_balance= d)? >> >> If it isn't, it would be nop regardless of putting cond_reshed at vm= scan.c. >> Because, although we complete zone balancing, kswapd doesn't sleep a= s >> pgdat_balance returns wrong result. And at last VM calls >> balance_pgdat. In this case, balance_pgdat returns without any work = as >> 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. >> >> But if your test was with Hanne's patch, I am very curious how come >> kswapd consumes CPU a lot. >> >> > The need_resched() in sleeping_prematurely() seems to be about the= best >> > option. =C2=A0The other option might be just to put a cond_resched= () in >> > kswapd_try_to_sleep(), but that will really have about the same ef= fect. >> >> I don't oppose it but before that, I think we have to know why kswap= d >> consumes CPU a lot although we applied Hannes' patch. >> > > Because it's still possible for processes to allocate pages at the sa= me > rate kswapd is freeing them leading to a situation where kswapd does = not > consider the zone balanced for prolonged periods of time. We have cond_resched in shrink_page_list, shrink_slab and balance_pgdat= =2E 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? > > -- > Mel Gorman > SUSE Labs > --=20 Kind regards, Minchan Kim -- 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