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Tue, 28 Mar 2023 11:35:43 -0700 (PDT) Received: from localhost ([2a02:8070:6387:ab20:5139:4abd:1194:8f0e]) by smtp.gmail.com with ESMTPSA id xh1-20020a170906da8100b0093f83562f09sm4460301ejb.87.2023.03.28.11.35.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 28 Mar 2023 11:35:43 -0700 (PDT) Date: Tue, 28 Mar 2023 14:35:41 -0400 From: Johannes Weiner To: Yosry Ahmed Cc: Tejun Heo , Josef Bacik , Jens Axboe , Zefan Li , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Michal =?iso-8859-1?Q?Koutn=FD?= , Vasily Averin , cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, bpf@vger.kernel.org Subject: Re: [PATCH v1 6/9] memcg: sleep during flushing stats in safe contexts Message-ID: References: <20230328061638.203420-1-yosryahmed@google.com> <20230328061638.203420-7-yosryahmed@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230328061638.203420-7-yosryahmed@google.com> X-Rspam-User: X-Rspamd-Queue-Id: B66771C0013 X-Rspamd-Server: rspam01 X-Stat-Signature: dubk4og9hfkkmkt5ccihbn93xgc1kph5 X-HE-Tag: 1680028544-102446 X-HE-Meta: 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 apu28m2f j6QxOtxW1YguX6jVoHXeY7C3kYzyNFvaoX24yQtIKGDvNw2jms2StdkAt1PujWJVcmeKayan8bUHf7rcVPk/4cl2Wtcqn1ewArAsQeDBjZffZfEgWZjGfX8gIHcXN9n8j/NrX657AQF3tE1S5PN3y1BeqpgQ/omb14qbelDBjbZxeV9D8g93pMhDvmh7XEx/WURwKv4Ex3+09WcTE4yvT1OvXDgGUNgLdsIfT5y5faLXbvkpJvyimfrQSNXstu7+KA1cLZFRvteDsoR2OcsSfKp3dLl6WtUnhStBi0G76S+a94pVba+vOEjdDC6ngesqo/TJO6/veo/YAit7onGMzAumc/CnRSsZIO0OwkC4E7gdo+R9ixfNWm6RdwRGCsg5kNDdeltVoQ7jYwengU6eHPiWuFVAXWmwoVessxgviFlc0VK/CysKkmF+DuGrv8WC+TJiS 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 Tue, Mar 28, 2023 at 06:16:35AM +0000, Yosry Ahmed wrote: > @@ -642,24 +642,57 @@ static void __mem_cgroup_flush_stats(void) > * from memcg flushers (e.g. reclaim, refault, etc). > */ > if (atomic_xchg(&stats_flush_ongoing, 1)) > - return; > + return false; > > WRITE_ONCE(flush_next_time, jiffies_64 + 2*FLUSH_TIME); > - cgroup_rstat_flush_atomic(root_mem_cgroup->css.cgroup); > + return true; > +} > + > +static void mem_cgroup_post_stats_flush(void) > +{ > atomic_set(&stats_flush_threshold, 0); > atomic_set(&stats_flush_ongoing, 0); > } > > -void mem_cgroup_flush_stats(void) > +static bool mem_cgroup_should_flush_stats(void) > { > - if (atomic_read(&stats_flush_threshold) > num_online_cpus()) > - __mem_cgroup_flush_stats(); > + return atomic_read(&stats_flush_threshold) > num_online_cpus(); > +} > + > +/* atomic functions, safe to call from any context */ > +static void __mem_cgroup_flush_stats_atomic(void) > +{ > + if (mem_cgroup_pre_stats_flush()) { > + cgroup_rstat_flush_atomic(root_mem_cgroup->css.cgroup); > + mem_cgroup_post_stats_flush(); > + } > +} I'm afraid I wasn't very nuanced with my complaint about the bool parameter in the previous version. In this case, when you can do a common helper for a couple of API functions defined right below it, and the callers don't spread throughout the codebase, using bools makes things simpler while still being easily understandable: static void do_flush_stats(bool may_sleep) { if (atomic_xchg(&stats_flush_ongoing, 1)) return; WRITE_ONCE(flush_next_time, jiffies_64 + 2*FLUSH_TIME); atomic_set(&stats_flush_threshold, 0); if (!may_sleep) cgroup_rstat_flush_atomic(root_mem_cgroup->css.cgroup); else cgroup_rstat_flush(root_mem_cgroup->css.cgroup); atomic_set(&stats_flush_ongoing, 0); } void mem_cgroup_flush_stats(void) { if (atomic_read(&stats_flush_threshold) > num_online_cpus()) do_flush_stats(true); } void mem_cgroup_flush_stats_atomic(void) { if (atomic_read(&stats_flush_threshold) > num_online_cpus()) do_flush_stats(false); } > void mem_cgroup_flush_stats_ratelimited(void) > { > if (time_after64(jiffies_64, READ_ONCE(flush_next_time))) > - mem_cgroup_flush_stats(); > + mem_cgroup_flush_stats_atomic(); > +} This should probably be mem_cgroup_flush_stats_atomic_ratelimited(). (Whee, kinda long, but that's alright. Very specialized caller...) Btw, can you guys think of a reason against moving the threshold check into the common function? It would then apply to the time-limited flushes as well, but that shouldn't hurt anything. This would make the code even simpler: static void do_flush_stats(bool may_sleep) { if (atomic_read(&stats_flush_threshold) <= num_online_cpus()) return; if (atomic_xchg(&stats_flush_ongoing, 1)) return; WRITE_ONCE(flush_next_time, jiffies_64 + 2*FLUSH_TIME); atomic_set(&stats_flush_threshold, 0); if (!may_sleep) cgroup_rstat_flush_atomic(root_mem_cgroup->css.cgroup); else cgroup_rstat_flush(root_mem_cgroup->css.cgroup); atomic_set(&stats_flush_ongoing, 0); } void mem_cgroup_flush_stats(void) { do_flush_stats(true); } void mem_cgroup_flush_stats_atomic(void) { do_flush_stats(false); } void mem_cgroup_flush_stats_atomic_ratelimited(void) { if (time_after64(jiffies_64, READ_ONCE(flush_next_time))) do_flush_stats(false); } > @@ -2845,7 +2845,7 @@ static void prepare_scan_count(pg_data_t *pgdat, struct scan_control *sc) > * Flush the memory cgroup stats, so that we read accurate per-memcg > * lruvec stats for heuristics. > */ > - mem_cgroup_flush_stats(); > + mem_cgroup_flush_stats_atomic(); I'm thinking this one could be non-atomic as well. It's called fairly high up in reclaim without any locks held.