From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lf0-f72.google.com (mail-lf0-f72.google.com [209.85.215.72]) by kanga.kvack.org (Postfix) with ESMTP id 6DA536B002E for ; Tue, 27 Mar 2018 05:18:55 -0400 (EDT) Received: by mail-lf0-f72.google.com with SMTP id f194-v6so6854860lff.6 for ; Tue, 27 Mar 2018 02:18:55 -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 q67-v6sor184215lfq.30.2018.03.27.02.18.53 for (Google Transport Security); Tue, 27 Mar 2018 02:18:54 -0700 (PDT) Date: Tue, 27 Mar 2018 12:18:50 +0300 From: Vladimir Davydov Subject: Re: [PATCH 02/10] mm: Maintain memcg-aware shrinkers in mcg_shrinkers array Message-ID: <20180327091850.ybql6l6gavhdieqg@esperanza> References: <152163840790.21546.980703278415599202.stgit@localhost.localdomain> <152163848990.21546.2153496613786165374.stgit@localhost.localdomain> <20180324184516.rogvydnnupr7ah2l@esperanza> <448bb904-a861-c2ae-0d3f-427e6a26f61e@virtuozzo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <448bb904-a861-c2ae-0d3f-427e6a26f61e@virtuozzo.com> Sender: owner-linux-mm@kvack.org List-ID: To: Kirill Tkhai 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 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.