From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f200.google.com (mail-qt0-f200.google.com [209.85.216.200]) by kanga.kvack.org (Postfix) with ESMTP id D33FC6B0008 for ; Tue, 3 Jul 2018 12:17:27 -0400 (EDT) Received: by mail-qt0-f200.google.com with SMTP id k8-v6so2465873qtj.18 for ; Tue, 03 Jul 2018 09:17:27 -0700 (PDT) Received: from EUR02-VE1-obe.outbound.protection.outlook.com (mail-eopbgr20110.outbound.protection.outlook.com. [40.107.2.110]) by mx.google.com with ESMTPS id q20-v6si1409115qtc.381.2018.07.03.09.17.26 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Tue, 03 Jul 2018 09:17:26 -0700 (PDT) Subject: Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker References: <153063036670.1818.16010062622751502.stgit@localhost.localdomain> <153063054586.1818.6041047871606697364.stgit@localhost.localdomain> <20180703152723.GB21590@bombadil.infradead.org> From: Kirill Tkhai Message-ID: Date: Tue, 3 Jul 2018 19:17:14 +0300 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Shakeel Butt , Matthew Wilcox Cc: 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 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. Kirill