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Fri, 23 Apr 2021 13:06:27 -0700 (PDT) From: Waiman Long X-Google-Original-From: Waiman Long Subject: Re: [PATCH-next v5 3/4] mm/memcg: Improve refill_obj_stock() performance To: Roman Gushchin , Waiman Long Cc: Johannes Weiner , Michal Hocko , Vladimir Davydov , Andrew Morton , Tejun Heo , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Vlastimil Babka , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, Shakeel Butt , Muchun Song , Alex Shi , Chris Down , Yafang Shao , Wei Yang , Masayoshi Mizuma , Xing Zhengjun , Matthew Wilcox References: <20210420192907.30880-1-longman@redhat.com> <20210420192907.30880-4-longman@redhat.com> Message-ID: <49715eb2-9ac4-8208-2c63-e432092c3ab2@redhat.com> Date: Fri, 23 Apr 2021 16:06:25 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.9.0 MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=llong@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 498D1E000122 X-Stat-Signature: z5dw58rppi9ydsagkxosk3drxwzirx4k Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf21; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=216.205.24.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1619208388-286856 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 4/22/21 10:28 PM, Roman Gushchin wrote: > On Thu, Apr 22, 2021 at 01:26:08PM -0400, Waiman Long wrote: >> On 4/21/21 7:55 PM, Roman Gushchin wrote: >>> On Tue, Apr 20, 2021 at 03:29:06PM -0400, Waiman Long wrote: >>>> There are two issues with the current refill_obj_stock() code. First of >>>> all, when nr_bytes reaches over PAGE_SIZE, it calls drain_obj_stock() to >>>> atomically flush out remaining bytes to obj_cgroup, clear cached_objcg >>>> and do a obj_cgroup_put(). It is likely that the same obj_cgroup will >>>> be used again which leads to another call to drain_obj_stock() and >>>> obj_cgroup_get() as well as atomically retrieve the available byte from >>>> obj_cgroup. That is costly. Instead, we should just uncharge the excess >>>> pages, reduce the stock bytes and be done with it. The drain_obj_stock() >>>> function should only be called when obj_cgroup changes. >>> I really like this idea! Thanks! >>> >>> However, I wonder if it can implemented simpler by splitting drain_obj_stock() >>> into two functions: >>> empty_obj_stock() will flush cached bytes, but not reset the objcg >>> drain_obj_stock() will call empty_obj_stock() and then reset objcg >>> >>> Then we simple can replace the second drain_obj_stock() in >>> refill_obj_stock() with empty_obj_stock(). What do you think? >> Actually the problem is the flushing cached bytes to objcg->nr_charged_bytes >> that can become a performance bottleneck in a multithreaded testing >> scenario. See my description in the latter half of my cover-letter. >> >> For cgroup v2, update the page charge will mostly update the per-cpu page >> charge stock. Flushing the remaining byte charge, however, will cause the >> obgcg to became the single contended cacheline for all the cpus that need to >> flush the byte charge. That is why I only update the page charge and left >> the remaining byte charge stayed put in the object stock. >> >>>> Secondly, when charging an object of size not less than a page in >>>> obj_cgroup_charge(), it is possible that the remaining bytes to be >>>> refilled to the stock will overflow a page and cause refill_obj_stock() >>>> to uncharge 1 page. To avoid the additional uncharge in this case, >>>> a new overfill flag is added to refill_obj_stock() which will be set >>>> when called from obj_cgroup_charge(). >>>> >>>> A multithreaded kmalloc+kfree microbenchmark on a 2-socket 48-core >>>> 96-thread x86-64 system with 96 testing threads were run. Before this >>>> patch, the total number of kilo kmalloc+kfree operations done for a 4k >>>> large object by all the testing threads per second were 4,304 kops/s >>>> (cgroup v1) and 8,478 kops/s (cgroup v2). After applying this patch, the >>>> number were 4,731 (cgroup v1) and 418,142 (cgroup v2) respectively. This >>>> represents a performance improvement of 1.10X (cgroup v1) and 49.3X >>>> (cgroup v2). >>> This part looks more controversial. Basically if there are N consequent >>> allocations of size (PAGE_SIZE + x), the stock will end up with (N * x) >>> cached bytes, right? It's not the end of the world, but do we really >>> need it given that uncharging a page is also cached? >> Actually the maximum charge that can be accumulated in (2*PAGE_SIZE + x - 1) >> since a following consume_obj_stock() will use those bytes once the byte >> charge is not less than (PAGE_SIZE + x). > Got it, thank you for the explanation! > > Can you, please, add a comment explaining what the "overfill" parameter does > and why it has different values on charge and uncharge paths? > Personally, I'd revert it's meaning and rename it to something like "trim" > or just plain "bool charge". > I think the simple explanation is that during the charge we can't refill more > than a PAGE_SIZE - 1 and the following allocation will likely use it or > the following deallocation will trim it if necessarily. > And on the uncharge path there are no bounds and the following deallocation > can only increase the cached value. Yes, that is the intention. I will make suggested change and put in a comment about it. Thanks, Longman