From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754889AbdCIOfh (ORCPT ); Thu, 9 Mar 2017 09:35:37 -0500 Received: from mx2.suse.de ([195.135.220.15]:50330 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753196AbdCIOeZ (ORCPT ); Thu, 9 Mar 2017 09:34:25 -0500 Date: Thu, 9 Mar 2017 14:20:44 +0000 From: Mel Gorman To: Johannes Weiner Cc: Michal Hocko , Minchan Kim , Andrew Morton , Jia He , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, Vlastimil Babka Subject: Re: [PATCH 1/9] mm: fix 100% CPU kswapd busyloop on unreclaimable nodes Message-ID: <20170309142044.5ewlvus6ana6boqp@suse.de> References: <20170228214007.5621-1-hannes@cmpxchg.org> <20170228214007.5621-2-hannes@cmpxchg.org> <20170303012609.GA3394@bbox> <20170303075954.GA31499@dhcp22.suse.cz> <20170306013740.GA8779@bbox> <20170306162410.GB2090@cmpxchg.org> <20170307101702.GD28642@dhcp22.suse.cz> <20170307165631.GA21425@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20170307165631.GA21425@cmpxchg.org> User-Agent: Mutt/1.6.2 (2016-07-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 07, 2017 at 11:56:31AM -0500, Johannes Weiner wrote: > On Tue, Mar 07, 2017 at 11:17:02AM +0100, Michal Hocko wrote: > > On Mon 06-03-17 11:24:10, Johannes Weiner wrote: > > > @@ -3271,7 +3271,8 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx) > > > * Raise priority if scanning rate is too low or there was no > > > * progress in reclaiming pages > > > */ > > > - if (raise_priority || !sc.nr_reclaimed) > > > + nr_reclaimed = sc.nr_reclaimed - nr_reclaimed; > > > + if (raise_priority || !nr_reclaimed) > > > sc.priority--; > > > } while (sc.priority >= 1); > > > > > > > I would rather not play with the sc state here. From a quick look at > > least > > /* > > * Fragmentation may mean that the system cannot be rebalanced for > > * high-order allocations. If twice the allocation size has been > > * reclaimed then recheck watermarks only at order-0 to prevent > > * excessive reclaim. Assume that a process requested a high-order > > * can direct reclaim/compact. > > */ > > if (sc->order && sc->nr_reclaimed >= compact_gap(sc->order)) > > sc->order = 0; > > > > does rely on the value. Wouldn't something like the following be safer? > > Well, what behavior is correct, though? This check looks like an > argument *against* resetting sc.nr_reclaimed. > > If kswapd is woken up for a higher order, this check sets a reclaim > cutoff beyond which it should give up on the order and balance for 0. > > That's on the scope of the kswapd invocation. Applying this threshold > to the outcome of just the preceeding priority seems like a mistake. > > Mel? Vlastimil? I cannot say which is definitely the correct behaviour. The current behaviour is conservative due to the historical concerns about kswapd reclaiming the world. The hazard as I see it is that resetting it *may* lead to more aggressive reclaim for high-order allocations. That may be a welcome outcome to some that really want high-order pages and be unwelcome to others that prefer pages to remain resident. However, in this case it's a tight window and problems would be tricky to detect. THP allocations won't trigger the behaviour and with vmalloc'd stack, I'd expect that only SLUB-intensive workloads using high-order pages would trigger any adverse behaviour. While I'm mildly concerned, I would be a little surprised if it actually caused runaway reclaim. -- Mel Gorman SUSE Labs