From: James Bottomley Subject: Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep Date: Mon, 16 May 2011 08:21:51 +0400 Message-ID: <1305519711.4806.7.camel@mulgrave.site> References: <1305295404-12129-1-git-send-email-mgorman@suse.de> <1305295404-12129-5-git-send-email-mgorman@suse.de> <4DCFAA80.7040109@jp.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: mgorman@suse.de, 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: KOSAKI Motohiro Return-path: Received: from bedivere.hansenpartnership.com ([66.63.167.143]:36020 "EHLO bedivere.hansenpartnership.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750787Ab1EPEWe (ORCPT ); Mon, 16 May 2011 00:22:34 -0400 In-Reply-To: <4DCFAA80.7040109@jp.fujitsu.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: 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 situation 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 > > --- > > mm/vmscan.c | 4 ++++ > > 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, > > unsigned long balanced = 0; > > bool all_zones_ok = true; > > > > + /* If kswapd has been running too long, just sleep */ > > + if (need_resched()) > > + 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: need_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 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 running without giving up the CPU. Generally that will mean we've been round the shrinker loop hundreds to thousands of times without sleeping. > It seems to be semi random behavior. Well, we have to do something. Chris Mason first suspected the hang was a kswapd rescheduling problem a while ago. We tried putting cond_rescheds() in several places in the vmscan code, but to no avail. The need_resched() in sleeping_prematurely() seems to be about the best option. The other option might be just to put a cond_resched() in kswapd_try_to_sleep(), but that will really have about the same effect. James