Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751777Ab1ERETk (ORCPT ); Wed, 18 May 2011 00:19:40 -0400 Received: from mail-qy0-f181.google.com ([209.85.216.181]:35407 "EHLO mail-qy0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750721Ab1ERETi convert rfc822-to-8bit (ORCPT ); Wed, 18 May 2011 00:19:38 -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=VPiO8SQr7NiLKUQYHiiOBQ3H/duk/XpxdSQuptSy7HYFqyzbOOexLT8TEvqTABK3bd P9cA9XT50vvQv+2RXjd4kBineiuVnn/ZwzkNSOa1ZaHpFoVTFLyT4leohFfb9L+4MMFT KB9k9yqYLPLTGM9UzThmNCJ/8MvTUeuITNH0k= MIME-Version: 1.0 In-Reply-To: <1305640239.2046.27.camel@lenovo> 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> <20110517103840.GL5279@suse.de> <1305640239.2046.27.camel@lenovo> Date: Wed, 18 May 2011 13:19:37 +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: Colin Ian King Cc: Mel Gorman , James Bottomley , KOSAKI Motohiro , akpm@linux-foundation.org, 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: 9783 Lines: 204 Hello Colin, On Tue, May 17, 2011 at 10:50 PM, Colin Ian King wrote: > On Tue, 2011-05-17 at 11:38 +0100, Mel Gorman wrote: >> On Tue, May 17, 2011 at 08:50:44AM +0900, 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; >> >  } >> > >> >> This makes some sense for the exit path but if one or more of the >> shrinkers takes a very long time without sleeping (extremely long >> list searches for example) then kswapd will not call cond_resched() >> between shrinkers and still consume a lot of CPU. >> >> > > >> > > 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. Okay. >> > It does make sense. >> > By your scenario, someone wakes up kswapd with higher order, again. >> > So re-enters balance_pgdat without ever have called cond_resched. >> > But if someone wakes up higher order again, we can't have a chance to >> > call kswapd_try_to_sleep. So your patch effect would be nop, too. >> > >> > It would be better to put cond_resched after balance_pgdat? >> > >> >> Which will leave kswapd runnable instead of going to sleep but >> guarantees a scheduling point. Lets see if the problem is that >> cond_resched is being missed although if this was the case then patch >> 4 would truly be a no-op but Colin has already reported that patch 1 on >> its own didn't fix his problem. If the problem is sandybridge-specific >> where kswapd remains runnable and consuming large amounts of CPU in >> turbo mode then we know that there are other cond_resched() decisions >> that will need to be revisited. >> >> Colin or James, would you be willing to test with patch 1 from this >> series and Minchan's patch below? Thanks. > > This works OK fine.  Ran 250 test cycles for about 2 hours. Thanks for the testing!. I would like to know exact patch for you to apply. My modification of inserting cond_resched is two. 1) shrink_slab function 2) kswapd right after balance_pgdat. 1) or 2) ? Or Both? Thanks -- 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/