Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753418Ab1EQAtD (ORCPT ); Mon, 16 May 2011 20:49:03 -0400 Received: from mail-qw0-f46.google.com ([209.85.216.46]:45092 "EHLO mail-qw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751633Ab1EQAtB convert rfc822-to-8bit (ORCPT ); Mon, 16 May 2011 20:49:01 -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=eBTIyRRkcVa9RA5E+I3SanA64898XPWm2aKtnpLehQjftwjSPKUY5Gl3uTwsWIVg5G lLkHC2IeRpIIzimVx/JY85ivpH+DfGjpGaKx/0ZjRnVEprkTSYkVZSiFdIKtWt54xUVH xTEujhrQxlBQ2uI+xyBDvrOLFrP7ggb968E0g= MIME-Version: 1.0 In-Reply-To: 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> <20110516102753.GF5279@suse.de> Date: Tue, 17 May 2011 09:48:59 +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: Mel Gorman 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 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: 7511 Lines: 161 On Tue, May 17, 2011 at 8:50 AM, Minchan Kim wrote: > On Mon, May 16, 2011 at 7:27 PM, Mel Gorman wrote: >> 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 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. >>> >> >>> > >>> > 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 not >>> > consider the zone balanced for prolonged periods of time. >>> >>> We have cond_resched in shrink_page_list, shrink_slab and balance_pgdat. >>> 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? >>> >> >> 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(). > > Don't we have to move cond_resched? > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 292582c..633e761 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -231,8 +231,10 @@ unsigned long shrink_slab(struct shrink_control *shrink, >        if (scanned == 0) >                scanned = SWAP_CLUSTER_MAX; > > -       if (!down_read_trylock(&shrinker_rwsem)) > -               return 1;       /* Assume we'll be able to shrink next time */ > +       if (!down_read_trylock(&shrinker_rwsem)) { > +               ret = 1; > +               goto out; /* Assume we'll be able to shrink next time */ > +       } > >        list_for_each_entry(shrinker, &shrinker_list, list) { >                unsigned long long delta; > @@ -280,12 +282,14 @@ unsigned long shrink_slab(struct shrink_control *shrink, >                        count_vm_events(SLABS_SCANNED, this_scan); >                        total_scan -= this_scan; > > -                       cond_resched(); >                } > >                shrinker->nr += total_scan; > +               cond_resched(); >        } >        up_read(&shrinker_rwsem); > +out: > +       cond_resched(); >        return ret; >  } > > >> >> 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(). > > If kswapd reclaims order-o followed by high order, it would have a > chance to call cond_resched in shrink_page_list. But if all zones are > all_unreclaimable is set, balance_pgdat could return any work. Typo : without any work. -- 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/