Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753463Ab1EQKis (ORCPT ); Tue, 17 May 2011 06:38:48 -0400 Received: from cantor.suse.de ([195.135.220.2]:60219 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753083Ab1EQKiq (ORCPT ); Tue, 17 May 2011 06:38:46 -0400 Date: Tue, 17 May 2011 11:38:40 +0100 From: Mel Gorman To: Minchan Kim 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 Subject: Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep Message-ID: <20110517103840.GL5279@suse.de> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 9271 Lines: 213 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. > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 292582c..61c45d0 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -2753,6 +2753,7 @@ static int kswapd(void *p) > if (!ret) { > trace_mm_vmscan_kswapd_wake(pgdat->node_id, order); > order = balance_pgdat(pgdat, order, &classzone_idx); > + cond_resched(); > } > } > return 0; > > > > > While it appears unlikely, there are bad conditions which can result > > in cond_resched() being avoided. > > > > > -- > > Mel Gorman > > SUSE Labs > > > > > > -- > Kind regards, > Minchan Kim -- Mel Gorman SUSE Labs -- 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/