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Thu, 14 Oct 2021 09:31:50 -0700 (PDT) Date: Thu, 14 Oct 2021 09:31:46 -0700 In-Reply-To: <20211013180130.GB22036@blackbody.suse.cz> Message-Id: <20211014163146.2177266-1-shakeelb@google.com> Mime-Version: 1.0 References: <20211013180130.GB22036@blackbody.suse.cz> X-Mailer: git-send-email 2.33.0.882.g93a45727a2-goog Subject: Re: [PATCH v2 1/2] memcg: flush stats only if updated From: Shakeel Butt To: mkoutny@suse.com Cc: akpm@linux-foundation.org, cgroups@vger.kernel.org, hannes@cmpxchg.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@kernel.org, shakeelb@google.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Authentication-Results: imf12.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=I3cPTF8S; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf12.hostedemail.com: domain of 3dltoYQgKCN8TIBLFFMCHPPHMF.DPNMJOVY-NNLWBDL.PSH@flex--shakeelb.bounces.google.com designates 209.85.210.201 as permitted sender) smtp.mailfrom=3dltoYQgKCN8TIBLFFMCHPPHMF.DPNMJOVY-NNLWBDL.PSH@flex--shakeelb.bounces.google.com X-Stat-Signature: ms3ckxb3exk4a9baattrr5ykadfbr4of X-Rspamd-Queue-Id: 49ADC10003D2 X-Rspamd-Server: rspam01 X-HE-Tag: 1634229111-1014 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: Hi Michal, On Wed, Oct 13, 2021 at 11:01 AM Michal Koutn=C3=BD wrot= e: > > On Fri, Oct 01, 2021 at 12:00:39PM -0700, Shakeel Butt wrote: > > In this patch we kept the stats update codepath very minimal and let th= e > > stats reader side to flush the stats only when the updates are over a > > specific threshold. =C2=A0For now the threshold is (nr_cpus * CHARGE_BA= TCH). > > BTW, a noob question -- are the updates always single page sized? > > This is motivated by apples vs oranges comparison since the > =C2=A0 =C2=A0 =C2=A0 =C2=A0 nr_cpus * MEMCG_CHARGE_BATCH > suggests what could the expected error be in pages (bytes). But it's most= ly > wrong since: a) uncertain single-page updates, b) various counter > updates summed together. I wonder whether the formula can serve to > provide at least some (upper) estimate. > Thanks for your review. This forces me to think more on this because each update does not necessarily be a single page sized update e.g. adding a hug= epage to an LRU. Though I think the error is time bounded by 2 seconds but in those 2 second= s mathematically the error can be large. What do you think of the following change? It will bound the error better within the 2 seconds window. >From e87a36eedd02b0d10d8f66f83833bd6e2bae17b8 Mon Sep 17 00:00:00 2001 From: Shakeel Butt Date: Thu, 14 Oct 2021 08:49:06 -0700 Subject: [PATCH] Better bounds on the stats error --- mm/memcontrol.c | 20 +++++++++++++------- 1 file changed, 13 insertions(+), 7 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 8f1d9c028897..e5d5c850a521 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -626,14 +626,20 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_= tree_per_node *mctz) static void flush_memcg_stats_dwork(struct work_struct *w); static DECLARE_DEFERRABLE_WORK(stats_flush_dwork, flush_memcg_stats_dwork)= ; static DEFINE_SPINLOCK(stats_flush_lock); -static DEFINE_PER_CPU(unsigned int, stats_updates); +static DEFINE_PER_CPU(int, stats_diff); static atomic_t stats_flush_threshold =3D ATOMIC_INIT(0); =20 -static inline void memcg_rstat_updated(struct mem_cgroup *memcg) +static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val) { + unsigned int x; + cgroup_rstat_updated(memcg->css.cgroup, smp_processor_id()); - if (!(__this_cpu_inc_return(stats_updates) % MEMCG_CHARGE_BATCH)) - atomic_inc(&stats_flush_threshold); + + x =3D abs(__this_cpu_add_return(stats_diff, val)); + if (x > MEMCG_CHARGE_BATCH) { + atomic_add(x / MEMCG_CHARGE_BATCH, &stats_flush_threshold); + __this_cpu_write(stats_diff, 0); + } } =20 static void __mem_cgroup_flush_stats(void) @@ -672,7 +678,7 @@ void __mod_memcg_state(struct mem_cgroup *memcg, int id= x, int val) return; =20 __this_cpu_add(memcg->vmstats_percpu->state[idx], val); - memcg_rstat_updated(memcg); + memcg_rstat_updated(memcg, val); } =20 /* idx can be of type enum memcg_stat_item or node_stat_item. */ @@ -705,7 +711,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, en= um node_stat_item idx, /* Update lruvec */ __this_cpu_add(pn->lruvec_stats_percpu->state[idx], val); =20 - memcg_rstat_updated(memcg); + memcg_rstat_updated(memcg, val); } =20 /** @@ -807,7 +813,7 @@ void __count_memcg_events(struct mem_cgroup *memcg, enu= m vm_event_item idx, return; =20 __this_cpu_add(memcg->vmstats_percpu->events[idx], count); - memcg_rstat_updated(memcg); + memcg_rstat_updated(memcg, val); } =20 static unsigned long memcg_events(struct mem_cgroup *memcg, int event) --=20 2.33.0.882.g93a45727a2-goog