From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756676AbdKOI4i (ORCPT ); Wed, 15 Nov 2017 03:56:38 -0500 Received: from mx2.suse.de ([195.135.220.15]:33834 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756532AbdKOI42 (ORCPT ); Wed, 15 Nov 2017 03:56:28 -0500 Date: Wed, 15 Nov 2017 09:56:25 +0100 From: Michal Hocko To: Minchan Kim Cc: Tetsuo Handa , Huang Ying , Mel Gorman , Vladimir Davydov , Johannes Weiner , Andrew Morton , Shakeel Butt , Greg Thelen , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] mm,vmscan: Kill global shrinker lock. Message-ID: <20171115085625.afvw333csgypbk24@dhcp22.suse.cz> References: <1510609063-3327-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> <20171115005602.GB23810@bbox> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171115005602.GB23810@bbox> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 15-11-17 09:56:02, Minchan Kim wrote: > On Tue, Nov 14, 2017 at 06:37:42AM +0900, Tetsuo Handa wrote: > > When shrinker_rwsem was introduced, it was assumed that > > register_shrinker()/unregister_shrinker() are really unlikely paths > > which are called during initialization and tear down. But nowadays, > > register_shrinker()/unregister_shrinker() might be called regularly. > > This patch prepares for allowing parallel registration/unregistration > > of shrinkers. > > > > Since do_shrink_slab() can reschedule, we cannot protect shrinker_list > > using one RCU section. But using atomic_inc()/atomic_dec() for each > > do_shrink_slab() call will not impact so much. > > > > This patch uses polling loop with short sleep for unregister_shrinker() > > rather than wait_on_atomic_t(), for we can save reader's cost (plain > > atomic_dec() compared to atomic_dec_and_test()), we can expect that > > do_shrink_slab() of unregistering shrinker likely returns shortly, and > > we can avoid khungtaskd warnings when do_shrink_slab() of unregistering > > shrinker unexpectedly took so long. > > > > Signed-off-by: Tetsuo Handa > > Before reviewing this patch, can't we solve the problem with more > simple way? Like this. > > Shakeel, What do you think? > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 13d711dd8776..cbb624cb9baa 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -498,6 +498,14 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid, > sc.nid = 0; > > freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible); > + /* > + * bail out if someone want to register a new shrinker to prevent > + * long time stall by parallel ongoing shrinking. > + */ > + if (rwsem_is_contended(&shrinker_rwsem)) { > + freed = 1; > + break; > + } So you want to do only partial slab shrinking if we have more contending direct reclaimers? This would just make a larger pressure on those on the list head rather than the tail. I do not think this is a good idea. -- Michal Hocko SUSE Labs From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f71.google.com (mail-wm0-f71.google.com [74.125.82.71]) by kanga.kvack.org (Postfix) with ESMTP id DCCCE6B0069 for ; Wed, 15 Nov 2017 03:56:28 -0500 (EST) Received: by mail-wm0-f71.google.com with SMTP id z3so379076wme.2 for ; Wed, 15 Nov 2017 00:56:28 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id 48si405433edz.287.2017.11.15.00.56.27 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 15 Nov 2017 00:56:27 -0800 (PST) Date: Wed, 15 Nov 2017 09:56:25 +0100 From: Michal Hocko Subject: Re: [PATCH 1/2] mm,vmscan: Kill global shrinker lock. Message-ID: <20171115085625.afvw333csgypbk24@dhcp22.suse.cz> References: <1510609063-3327-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> <20171115005602.GB23810@bbox> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171115005602.GB23810@bbox> Sender: owner-linux-mm@kvack.org List-ID: To: Minchan Kim Cc: Tetsuo Handa , Huang Ying , Mel Gorman , Vladimir Davydov , Johannes Weiner , Andrew Morton , Shakeel Butt , Greg Thelen , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Wed 15-11-17 09:56:02, Minchan Kim wrote: > On Tue, Nov 14, 2017 at 06:37:42AM +0900, Tetsuo Handa wrote: > > When shrinker_rwsem was introduced, it was assumed that > > register_shrinker()/unregister_shrinker() are really unlikely paths > > which are called during initialization and tear down. But nowadays, > > register_shrinker()/unregister_shrinker() might be called regularly. > > This patch prepares for allowing parallel registration/unregistration > > of shrinkers. > > > > Since do_shrink_slab() can reschedule, we cannot protect shrinker_list > > using one RCU section. But using atomic_inc()/atomic_dec() for each > > do_shrink_slab() call will not impact so much. > > > > This patch uses polling loop with short sleep for unregister_shrinker() > > rather than wait_on_atomic_t(), for we can save reader's cost (plain > > atomic_dec() compared to atomic_dec_and_test()), we can expect that > > do_shrink_slab() of unregistering shrinker likely returns shortly, and > > we can avoid khungtaskd warnings when do_shrink_slab() of unregistering > > shrinker unexpectedly took so long. > > > > Signed-off-by: Tetsuo Handa > > Before reviewing this patch, can't we solve the problem with more > simple way? Like this. > > Shakeel, What do you think? > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 13d711dd8776..cbb624cb9baa 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -498,6 +498,14 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid, > sc.nid = 0; > > freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible); > + /* > + * bail out if someone want to register a new shrinker to prevent > + * long time stall by parallel ongoing shrinking. > + */ > + if (rwsem_is_contended(&shrinker_rwsem)) { > + freed = 1; > + break; > + } So you want to do only partial slab shrinking if we have more contending direct reclaimers? This would just make a larger pressure on those on the list head rather than the tail. I do not think this is a good idea. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org