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[70.44.39.90]) by smtp.gmail.com with ESMTPSA id o23sm2348385qka.16.2021.04.15.09.50.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 15 Apr 2021 09:50:29 -0700 (PDT) Date: Thu, 15 Apr 2021 12:50:28 -0400 From: Johannes Weiner To: Waiman Long Cc: Michal Hocko , Vladimir Davydov , Andrew Morton , Tejun Heo , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Vlastimil Babka , Roman Gushchin , 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 Subject: Re: [PATCH v3 3/5] mm/memcg: Cache vmstat data in percpu memcg_stock_pcp Message-ID: References: <20210414012027.5352-1-longman@redhat.com> <20210414012027.5352-4-longman@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210414012027.5352-4-longman@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 13, 2021 at 09:20:25PM -0400, Waiman Long wrote: > Before the new slab memory controller with per object byte charging, > charging and vmstat data update happen only when new slab pages are > allocated or freed. Now they are done with every kmem_cache_alloc() > and kmem_cache_free(). This causes additional overhead for workloads > that generate a lot of alloc and free calls. > > The memcg_stock_pcp is used to cache byte charge for a specific > obj_cgroup to reduce that overhead. To further reducing it, this patch > makes the vmstat data cached in the memcg_stock_pcp structure as well > until it accumulates a page size worth of update or when other cached > data change. > > On a 2-socket Cascade Lake server with instrumentation enabled and this > patch applied, it was found that about 17% (946796 out of 5515184) of the > time when __mod_obj_stock_state() is called leads to an actual call to > mod_objcg_state() after initial boot. When doing parallel kernel build, > the figure was about 16% (21894614 out of 139780628). So caching the > vmstat data reduces the number of calls to mod_objcg_state() by more > than 80%. Right, but mod_objcg_state() is itself already percpu-cached. What's the benefit of avoiding calls to it with another percpu cache?