From: Colin Ian King Subject: Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep Date: Tue, 17 May 2011 14:50:39 +0100 Message-ID: <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> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: Minchan Kim , 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 To: Mel Gorman Return-path: In-Reply-To: <20110517103840.GL5279@suse.de> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org 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. > > > 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 >