From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f72.google.com (mail-wm0-f72.google.com [74.125.82.72]) by kanga.kvack.org (Postfix) with ESMTP id 799BB6B000C for ; Tue, 3 Jul 2018 13:00:46 -0400 (EDT) Received: by mail-wm0-f72.google.com with SMTP id v13-v6so1013818wmc.1 for ; Tue, 03 Jul 2018 10:00:46 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id 125-v6sor608886wmk.50.2018.07.03.10.00.44 for (Google Transport Security); Tue, 03 Jul 2018 10:00:44 -0700 (PDT) MIME-Version: 1.0 References: <153063036670.1818.16010062622751502.stgit@localhost.localdomain> <153063054586.1818.6041047871606697364.stgit@localhost.localdomain> <20180703152723.GB21590@bombadil.infradead.org> In-Reply-To: From: Shakeel Butt Date: Tue, 3 Jul 2018 10:00:32 -0700 Message-ID: Subject: Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Kirill Tkhai Cc: Matthew Wilcox , Vladimir Davydov , Alexander Viro , Johannes Weiner , Michal Hocko , Thomas Gleixner , Philippe Ombredanne , stummala@codeaurora.org, gregkh@linuxfoundation.org, Stephen Rothwell , Roman Gushchin , mka@chromium.org, Tetsuo Handa , Chris Wilson , longman@redhat.com, Minchan Kim , Huang Ying , Mel Gorman , jbacik@fb.com, Guenter Roeck , LKML , Linux MM , lirongqing@baidu.com, Andrey Ryabinin , Andrew Morton , Paul McKenney On Tue, Jul 3, 2018 at 9:17 AM Kirill Tkhai wrote: > > Hi, Shakeel, > > On 03.07.2018 18:46, Shakeel Butt wrote: > > On Tue, Jul 3, 2018 at 8:27 AM Matthew Wilcox wrote: > >> > >> On Tue, Jul 03, 2018 at 06:09:05PM +0300, Kirill Tkhai wrote: > >>> +++ b/mm/vmscan.c > >>> @@ -169,6 +169,49 @@ unsigned long vm_total_pages; > >>> static LIST_HEAD(shrinker_list); > >>> static DECLARE_RWSEM(shrinker_rwsem); > >>> > >>> +#ifdef CONFIG_MEMCG_KMEM > >>> +static DEFINE_IDR(shrinker_idr); > >>> +static int shrinker_nr_max; > >> > >> So ... we've now got a list_head (shrinker_list) which contains all of > >> the shrinkers, plus a shrinker_idr which contains the memcg-aware shrinkers? > >> > >> Why not replace the shrinker_list with the shrinker_idr? It's only used > >> twice in vmscan.c: > >> > >> void register_shrinker_prepared(struct shrinker *shrinker) > >> { > >> down_write(&shrinker_rwsem); > >> list_add_tail(&shrinker->list, &shrinker_list); > >> up_write(&shrinker_rwsem); > >> } > >> > >> list_for_each_entry(shrinker, &shrinker_list, list) { > >> ... > >> > >> The first is simply idr_alloc() and the second is > >> > >> idr_for_each_entry(&shrinker_idr, shrinker, id) { > >> > >> I understand there's a difference between allocating the shrinker's ID and > >> adding it to the list. You can do this by calling idr_alloc with NULL > >> as the pointer, and then using idr_replace() when you want to add the > >> shrinker to the list. idr_for_each_entry() skips over NULL entries. > >> > >> This will actually reduce the size of each shrinker and be more > >> cache-efficient when calling the shrinkers. I think we can also get > >> rid of the shrinker_rwsem eventually, but let's leave it for now. > > > > Can you explain how you envision shrinker_rwsem can be removed? I am > > very much interested in doing that. > > Have you tried to do some games with SRCU? It looks like we just need to > teach count_objects() and scan_objects() to work with semi-destructed > shrinkers. Though, this looks this will make impossible to introduce > shrinkers, which do synchronize_srcu() in scan_objects() for example. > Not sure, someone will actually use this, and this is possible to consider > as limitation. > Hi Kirill, I tried SRCU and the discussion is at https://lore.kernel.org/lkml/20171117173521.GA21692@infradead.org/T/#u Paul E. McKenney suggested to enable SRCU unconditionally. So, to use SRCU for shrinkers, we first have to push unconditional SRCU. Tetsuo had another lockless solution which was a bit involved but does not depend on SRCU. thanks, Shakeel