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From: Shakeel Butt To: Yosry Ahmed Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , ying.huang@intel.com, feng.tang@intel.com, fengwei.yin@intel.com, oliver.sang@intel.com, kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] memcg: rearrage fields of mem_cgroup_per_node Message-ID: References: <20240523034824.1255719-1-shakeel.butt@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Stat-Signature: cww43wzuid43mnackjkwyf7gbsgdpnjq X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 4422D1A0015 X-HE-Tag: 1716442486-542378 X-HE-Meta: U2FsdGVkX19WLLJ4ylL0J6RCJbTMmBbEH4QllnU5nv5KGK1nyOKUBGkseMz83YM9Bw8isG2nPK9A0P6176FNVj2E9wacdW0jpkpG36P5sMm+CNsRx01MZAyJxqx3Sh7HxqkIfdEIZkQXY9l8eDeIAgWZWKtJB9/t81btgfuN2jAKtvYsxt7i3I3woEYbJbQUj7Z5NHmGa3ELKvKvIorL38++riNPY6Iz2/pRDaSvo8wPOsbsC/tfaLodClTndPQiOz/wBPFd0nypFnyCORkeB1/h06ZeO6xH8uEoEFIxJtJRCrZQ6UvtzWObfe9iPprk5n/DgxlbjWhroShNzG1E6g2BAZy3DfZTAc4zAUVGbTZlpkkmXWWKDAQpn2ZDxDbkG2ZZWn43CA01FbsmMFinOncg0LlkhlCLt6isIxrFxdjy3tvfCKhzsTmnATcYhuZewjTL02rG5uCvCTGzX60GMcFHD/INOQASZ7zWFnXn94RVQOtN4hGnSjEmuxj3eT9U9fraOBWB17ONvYDaeyHfPJYQBGy17eTIF5MUjWTCRAvIcKej17/9w2knlqb03QQirRe2yBmnYUeTy64glTniKfuEIuzMtTB0JGUTgslZKxvdTqkRHw2asvS+LrxIXZ3XVPdPQj49NlV1JCo169+ieMBi3nWHo0929/A3jCuHjWa7UzhWthy3TFkdY4Z0o2se+Yr/DDMiAIkzh4zXbOArdF2SgbgbonePmqkDiT1i/Y7q1HVt5T7pKqV8KEosfJDN4q0aGQauO3Rr2XBa4fzvyg3IV+rQkocjq0Ujn13KY2ZU3S1GmTzy/dZsL8l31mdAzrW8IuYZ1YL3UmTa0F9LmltcCvXhPREPi/SAqmgC8Re2qWH226yvx6Rm/s3hwerRjcMGJ0E/hfcEfHgVo/w720GO6DVKV4twnO3q5teFT8e3/EjObhaN7nIws2D3Pt0ga2rvEyKc06YPlr3TFj+ qNCgvVt5 dIu4MvCszjxe1MHn2gRMRnn2bGYcgQeYj7kMrsT7aOUONKA5W8+j0XGbjqTYUTsl2l3F+wBAo6EB7NWO9R3N4/HLNKZQz9L0llHlHNtJEmK5bVogT2BOodxDbkoyFtI6HZmxWqPvv1pykJSDkjEITg4K8swrld+ES/hPn 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, May 22, 2024 at 09:35:57PM -0700, Yosry Ahmed wrote: > On Wed, May 22, 2024 at 8:48 PM Shakeel Butt wrote: [...] > > > > struct mem_cgroup_per_node { > > - struct lruvec lruvec; > > + /* Keep the read-only fields at the start */ > > + struct mem_cgroup *memcg; /* Back pointer, we cannot */ > > + /* use container_of */ > > > > struct lruvec_stats_percpu __percpu *lruvec_stats_percpu; > > struct lruvec_stats *lruvec_stats; > > - > > - unsigned long lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS]; > > - > > - struct mem_cgroup_reclaim_iter iter; > > - > > struct shrinker_info __rcu *shrinker_info; > > > > + /* memcg-v1 only stuff in middle */ > > + > > struct rb_node tree_node; /* RB tree node */ > > unsigned long usage_in_excess;/* Set to the value by which */ > > /* the soft limit is exceeded*/ > > bool on_tree; > > - struct mem_cgroup *memcg; /* Back pointer, we cannot */ > > - /* use container_of */ > > Do we need CACHELINE_PADDING() here (or maybe make struct lruvec > cache-aligned) to make sure the false cacheline sharing doesn't happen > again with the fields below, or is the idea that the fields that get > read in hot paths (memcg, lruvec_stats_percpu, lruvec_stats) are far > at the top, and the memcg v1 elements in the middle act as a buffer? > > IOW, is sharing between the fields below and memcg v1 fields okay > because they are not read in the hot path? If yes, I believe it's > worth a comment. It can be easily missed if the memcg v1 soft limit is > removed later for example. > For 6.10, I wanted to keep the change simple and yes, the memcg v1 stuff as a buffer between the pointers and lruvec/lru_zone_size fields. For 6.11 or later kernels, I am planning to use some asserts to make sure these fields don't share a cacheline, so later when we remove the v1-only stuff, the asserts will make sure we keep the separate cacheline property intact.