From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f72.google.com (mail-pl0-f72.google.com [209.85.160.72]) by kanga.kvack.org (Postfix) with ESMTP id 47D2C6B0271 for ; Tue, 3 Jul 2018 13:32:38 -0400 (EDT) Received: by mail-pl0-f72.google.com with SMTP id x23-v6so1534677pln.11 for ; Tue, 03 Jul 2018 10:32:38 -0700 (PDT) Received: from EUR04-VI1-obe.outbound.protection.outlook.com (mail-eopbgr80124.outbound.protection.outlook.com. [40.107.8.124]) by mx.google.com with ESMTPS id 43-v6si1552441pla.362.2018.07.03.10.32.36 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Tue, 03 Jul 2018 10:32:36 -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 20:32:25 +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 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 03.07.2018 20:00, Shakeel Butt wrote: > 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. First time, I read this, I though the talk goes about some new srcu_read_lock() without an argument and it's need to rework SRCU in some huge way. Thanks god, it was just a misreading :) > Tetsuo had another lockless solution which was a bit involved but does > not depend on SRCU. Ok, I see refcounters suggestion. Thanks for the link, Shakeel! Kirill