Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751327Ab1EPFEE (ORCPT ); Mon, 16 May 2011 01:04:04 -0400 Received: from mail-qy0-f174.google.com ([209.85.216.174]:37310 "EHLO mail-qy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751129Ab1EPFEC convert rfc822-to-8bit (ORCPT ); Mon, 16 May 2011 01:04:02 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=FJQGUUcMI40UtjMbSHLbRRjz015uJAtNSD0JH7U1H2QrZQLs4TS6ItT2k0CD69mNTm RJO8SpaxqIusUbm+r7CyDQIXTxeRzCic8vm1gRAFiqvDbN25TjbM2is+1DIrSjIRvXqG RtXNNdUfrICtnFYI6cohQvMnAQKC1eWlJB0h0= MIME-Version: 1.0 In-Reply-To: <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> <1305519711.4806.7.camel@mulgrave.site> Date: Mon, 16 May 2011 14:04:00 +0900 Message-ID: Subject: Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep From: Minchan Kim To: James Bottomley Cc: KOSAKI Motohiro , 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 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3560 Lines: 90 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 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. Is it a result of test 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 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.  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. 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. > > James > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org.  For more info on Linux MM, > see: http://www.linux-mm.org/ . > Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ > Don't email: email@kvack.org > -- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/