Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751303AbdH1NId (ORCPT ); Mon, 28 Aug 2017 09:08:33 -0400 Received: from mx2.suse.de ([195.135.220.15]:52362 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751170AbdH1NIc (ORCPT ); Mon, 28 Aug 2017 09:08:32 -0400 Date: Mon, 28 Aug 2017 15:08:29 +0200 From: Michal Hocko To: Vlastimil Babka Cc: js1304@gmail.com, Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Mel Gorman Subject: Re: [PATCH 2/2] mm/slub: don't use reserved highatomic pageblock for optimistic try Message-ID: <20170828130829.GL17097@dhcp22.suse.cz> References: <1503882675-17910-1-git-send-email-iamjoonsoo.kim@lge.com> <1503882675-17910-2-git-send-email-iamjoonsoo.kim@lge.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2996 Lines: 69 On Mon 28-08-17 13:29:29, Vlastimil Babka wrote: > On 08/28/2017 03:11 AM, js1304@gmail.com wrote: > > From: Joonsoo Kim > > > > High-order atomic allocation is difficult to succeed since we cannot > > reclaim anything in this context. So, we reserves the pageblock for > > this kind of request. > > > > In slub, we try to allocate higher-order page more than it actually > > needs in order to get the best performance. If this optimistic try is > > used with GFP_ATOMIC, alloc_flags will be set as ALLOC_HARDER and > > the pageblock reserved for high-order atomic allocation would be used. > > Moreover, this request would reserve the MIGRATE_HIGHATOMIC pageblock > > ,if succeed, to prepare further request. It would not be good to use > > MIGRATE_HIGHATOMIC pageblock in terms of fragmentation management > > since it unconditionally set a migratetype to request's migratetype > > when unreserving the pageblock without considering the migratetype of > > used pages in the pageblock. > > > > This is not what we don't intend so fix it by unconditionally setting > > __GFP_NOMEMALLOC in order to not set ALLOC_HARDER. > > I wonder if it would be more robust to strip GFP_ATOMIC from alloc_gfp. > E.g. __GFP_NOMEMALLOC does seem to prevent ALLOC_HARDER, but not > ALLOC_HIGH. Or maybe we should adjust __GFP_NOMEMALLOC implementation > and document it more thoroughly? CC Michal Hocko Yeah, __GFP_NOMEMALLOC is rather inconsistent. It has been added to override __GFP_MEMALLOC resp. PF_MEMALLOC AFAIK. In this particular case I would agree that dropping __GFP_HIGH and __GFP_ATOMIC would be more precise. I am not sure we want to touch the existing semantic of __GFP_NOMEMALLOC though. This would require auditing all the existing users (something tells me that quite some of those will be incorrect...) > Also, were these 2 patches done via code inspection or you noticed > suboptimal behavior which got fixed? Thanks. The patch description is not very clear to me either but I guess that Joonsoo sees to many larger order pages to back slab objects when the system is not under heavy memory pressure and that increases internal fragmentation? > > Signed-off-by: Joonsoo Kim > > --- > > mm/slub.c | 6 ++---- > > 1 file changed, 2 insertions(+), 4 deletions(-) > > > > diff --git a/mm/slub.c b/mm/slub.c > > index e1e442c..fd8dd89 100644 > > --- a/mm/slub.c > > +++ b/mm/slub.c > > @@ -1579,10 +1579,8 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node) > > */ > > alloc_gfp = (flags | __GFP_NOWARN | __GFP_NORETRY) & ~__GFP_NOFAIL; > > if (oo_order(oo) > oo_order(s->min)) { > > - if (alloc_gfp & __GFP_DIRECT_RECLAIM) { > > - alloc_gfp |= __GFP_NOMEMALLOC; > > - alloc_gfp &= ~__GFP_DIRECT_RECLAIM; > > - } > > + alloc_gfp |= __GFP_NOMEMALLOC; > > + alloc_gfp &= ~__GFP_DIRECT_RECLAIM; > > } > > > > page = alloc_slab_page(s, alloc_gfp, node, oo); > > -- Michal Hocko SUSE Labs