From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f199.google.com (mail-pf0-f199.google.com [209.85.192.199]) by kanga.kvack.org (Postfix) with ESMTP id 5E8EA6B0010 for ; Tue, 27 Mar 2018 11:30:41 -0400 (EDT) Received: by mail-pf0-f199.google.com with SMTP id k17so13185144pfj.10 for ; Tue, 27 Mar 2018 08:30:41 -0700 (PDT) Received: from EUR02-VE1-obe.outbound.protection.outlook.com (mail-eopbgr20096.outbound.protection.outlook.com. [40.107.2.96]) by mx.google.com with ESMTPS id r13si1024825pgp.504.2018.03.27.08.30.37 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Tue, 27 Mar 2018 08:30:38 -0700 (PDT) Subject: Re: [PATCH 02/10] mm: Maintain memcg-aware shrinkers in mcg_shrinkers array References: <152163840790.21546.980703278415599202.stgit@localhost.localdomain> <152163848990.21546.2153496613786165374.stgit@localhost.localdomain> <20180324184516.rogvydnnupr7ah2l@esperanza> <448bb904-a861-c2ae-0d3f-427e6a26f61e@virtuozzo.com> <20180327091850.ybql6l6gavhdieqg@esperanza> From: Kirill Tkhai Message-ID: <6660e4c1-7a52-9203-e901-fc87be328c8b@virtuozzo.com> Date: Tue, 27 Mar 2018 18:30:22 +0300 MIME-Version: 1.0 In-Reply-To: <20180327091850.ybql6l6gavhdieqg@esperanza> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Vladimir Davydov Cc: viro@zeniv.linux.org.uk, hannes@cmpxchg.org, mhocko@kernel.org, akpm@linux-foundation.org, tglx@linutronix.de, pombredanne@nexb.com, stummala@codeaurora.org, gregkh@linuxfoundation.org, sfr@canb.auug.org.au, guro@fb.com, mka@chromium.org, penguin-kernel@I-love.SAKURA.ne.jp, chris@chris-wilson.co.uk, longman@redhat.com, minchan@kernel.org, hillf.zj@alibaba-inc.com, ying.huang@intel.com, mgorman@techsingularity.net, shakeelb@google.com, jbacik@fb.com, linux@roeck-us.net, linux-kernel@vger.kernel.org, linux-mm@kvack.org, willy@infradead.org On 27.03.2018 12:18, Vladimir Davydov wrote: > On Mon, Mar 26, 2018 at 06:20:55PM +0300, Kirill Tkhai wrote: >> On 24.03.2018 21:45, Vladimir Davydov wrote: >>> On Wed, Mar 21, 2018 at 04:21:29PM +0300, Kirill Tkhai wrote: >>>> The patch introduces mcg_shrinkers array to keep memcg-aware >>>> shrinkers in order of their shrinker::id. >>>> >>>> This allows to access the shrinkers dirrectly by the id, >>>> without iteration over shrinker_list list. >>> >>> Why don't you simply use idr instead of ida? With idr you wouldn't need >>> the array mapping shrinker id to shrinker ptr. AFAIU you need this >>> mapping to look up the shrinker by id in shrink_slab. The latter doesn't >>> seem to be a hot path so using idr there should be acceptable. Since we >>> already have shrinker_rwsem, which is taken for reading by shrink_slab, >>> we wouldn't even need any additional locking for it. >> >> The reason is ida may allocate memory, and since list_lru_add() can't fail, >> we can't do that there. If we allocate all the ida memory at the time of >> memcg creation (i.e., preallocate it), this is not different to the way >> the bitmap makes. >> >> While bitmap has the agvantage, since it's simplest data structure (while >> ida has some radix tree overhead). >> >> Also, bitmap does not require a lock, there is single atomic operation >> to set or clear a bit, and it scales better, when anything. > > I didn't mean the per-memcg bitmaps - I think it's OK to use plain > arrays for them and reallocate them with the aid of RCU. > > What I actually mean is the mapping shrink_id => shrinker. AFAIU it > isn't accessed from list_lru, it is only needed to look up a shrinker > by id from shrink_slab(). The latter is rather a slow path so I think > we can use an IDR for this mapping instead of IDA + plain array. This is good idea. Thanks, Kirill