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From: Roman Gushchin To: Yosry Ahmed Cc: Johannes Weiner , Michal Hocko , Shakeel Butt , Muchun Song , Andrew Morton , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2] mm: memcg: use rstat for non-hierarchical stats Message-ID: References: <20230726002904.655377-1-yosryahmed@google.com> <20230726002904.655377-2-yosryahmed@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230726002904.655377-2-yosryahmed@google.com> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 31AB220012 X-Stat-Signature: nfoouzbjjtz14ptt1dxhwbcbkhsreiro X-HE-Tag: 1690337753-365768 X-HE-Meta: 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 O4YW7+Q8 DIewe6xMSfh2W8A5g1qcb4ldGXEWOpMa0uQawaf1XkfOwwyZF6t5dpQBoY/uFAtGmW5GVnz3MboWhCigFToMvaEybnva2SceOfT5l1y5wUEfwLDJwEilxB0cGRmqkhbUkV5CO0713HqcsqeQ8cQvWx+O17YqDF4gu1D2YzmscP2xecPO4gPOOQ6+WhDBlcoa4AErNWnn+R3Zy8ozVN6xXQp9mnDT2/CIbw9a5 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 Wed, Jul 26, 2023 at 12:29:04AM +0000, Yosry Ahmed wrote: > Currently, memcg uses rstat to maintain hierarchical stats. Counters are > maintained for hierarchical stats at each memcg. Rstat tracks which > cgroups have updates on which cpus to keep those counters fresh on the > read-side. > > For non-hierarchical stats, we do not maintain counters. Instead, the global? > percpu counters for a given stat need to be summed to get the > non-hierarchical stat value. The original implementation did the same. > At some point before rstat, non-hierarchical counters were introduced by > commit a983b5ebee57 ("mm: memcontrol: fix excessive complexity in > memory.stat reporting"). However, those counters were updated on the > performance critical write-side, which caused regressions, so they were > later removed by commit 815744d75152 ("mm: memcontrol: don't batch > updates of local VM stats and events"). See [1] for more detailed > history. > > Kernel versions in between a983b5ebee57 & 815744d75152 (a year and a > half) enjoyed cheap reads of non-hierarchical stats, specifically on > cgroup v1. When moving to more recent kernels, a performance regression > for reading non-hierarchical stats is observed. > > Now that we have rstat, we know exactly which percpu counters have > updates for each stat. We can maintain non-hierarchical counters again, > making reads much more efficient, without affecting the performance > critical write-side. Hence, add non-hierarchical (i.e local) counters > for the stats, and extend rstat flushing to keep those up-to-date. > > A caveat is that we now a stats flush before reading need? > local/non-hierarchical stats through {memcg/lruvec}_page_state_local() > or memcg_events_local(), where we previously only needed a flush to > read hierarchical stats. Most contexts reading non-hierarchical stats > are already doing a flush, add a flush to the only missing context in > count_shadow_nodes(). > > With this patch, reading memory.stat from 1000 memcgs is 3x faster on a > machine with 256 cpus on cgroup v1: > # for i in $(seq 1000); do mkdir /sys/fs/cgroup/memory/cg$i; done > # time cat /dev/cgroup/memory/cg*/memory.stat > /dev/null > real 0m0.125s > user 0m0.005s > sys 0m0.120s > > After: > real 0m0.032s > user 0m0.005s > sys 0m0.027s > > [1]https://lore.kernel.org/lkml/20230725201811.GA1231514@cmpxchg.org/ > > Signed-off-by: Yosry Ahmed > Acked-by: Johannes Weiner Acked-by: Roman Gushchin Thank you!