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Fri, 26 Jan 2024 07:36:35 -0800 (PST) Received: from localhost ([2620:10d:c091:400::5:271e]) by smtp.gmail.com with ESMTPSA id on6-20020a056214448600b006819a4354basm601263qvb.37.2024.01.26.07.36.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 26 Jan 2024 07:36:35 -0800 (PST) Date: Fri, 26 Jan 2024 10:36:34 -0500 From: Johannes Weiner To: Yosry Ahmed Cc: Andrew Morton , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel test robot Subject: Re: [PATCH] mm: memcg: optimize parent iteration in memcg_rstat_updated() Message-ID: <20240126153634.GH1567330@cmpxchg.org> References: <20240124100023.660032-1-yosryahmed@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240124100023.660032-1-yosryahmed@google.com> X-Rspamd-Queue-Id: DF7F5180002 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 8co9u7sp9zsws755gij3c3m7jnxxytkc X-HE-Tag: 1706283396-858336 X-HE-Meta: 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 fpvYd01t 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 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: List-Subscribe: List-Unsubscribe: On Wed, Jan 24, 2024 at 10:00:22AM +0000, Yosry Ahmed wrote: > In memcg_rstat_updated(), we iterate the memcg being updated and its > parents to update memcg->vmstats_percpu->stats_updates in the fast path > (i.e. no atomic updates). According to my math, this is 3 memory loads > (and potentially 3 cache misses) per memcg: > - Load the address of memcg->vmstats_percpu. > - Load vmstats_percpu->stats_updates (based on some percpu calculation). > - Load the address of the parent memcg. > > Avoid most of the cache misses by caching a pointer from each struct > memcg_vmstats_percpu to its parent on the corresponding CPU. In this > case, for the first memcg we have 2 memory loads (same as above): > - Load the address of memcg->vmstats_percpu. > - Load vmstats_percpu->stats_updates (based on some percpu calculation). > > Then for each additional memcg, we need a single load to get the > parent's stats_updates directly. This reduces the number of loads from > O(3N) to O(2+N) -- where N is the number of memcgs we need to iterate. > > Additionally, stash a pointer to memcg->vmstats in each struct > memcg_vmstats_percpu such that we can access the atomic counter that all > CPUs fold into, memcg->vmstats->stats_updates. > memcg_should_flush_stats() is changed to memcg_vmstats_needs_flush() to > accept a struct memcg_vmstats pointer accordingly. > > In struct memcg_vmstats_percpu, make sure both pointers together with > stats_updates live on the same cacheline. Finally, update > mem_cgroup_alloc() to take in a parent pointer and initialize the new > cache pointers on each CPU. The percpu loop in mem_cgroup_alloc() may > look concerning, but there are multiple similar loops in the cgroup > creation path (e.g. cgroup_rstat_init()), most of which are hidden > within alloc_percpu(). > > According to Oliver's testing [1], this fixes multiple 30-38% > regressions in vm-scalability, will-it-scale-tlb_flush2, and > will-it-scale-fallocate1. This comes at a cost of 2 more pointers per > CPU (<2KB on a machine with 128 CPUs). > > [1] https://lore.kernel.org/lkml/ZbDJsfsZt2ITyo61@xsang-OptiPlex-9020/ > > Fixes: 8d59d2214c23 ("mm: memcg: make stats flushing threshold per-memcg") > Tested-by: kernel test robot > Reported-by: kernel test robot > Closes: https://lore.kernel.org/oe-lkp/202401221624.cb53a8ca-oliver.sang@intel.com > Signed-off-by: Yosry Ahmed Nice! Acked-by: Johannes Weiner