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[70.44.39.90]) by smtp.gmail.com with ESMTPSA id p16sm1025148qtq.24.2021.02.08.12.29.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 08 Feb 2021 12:29:21 -0800 (PST) Date: Mon, 8 Feb 2021 15:29:21 -0500 From: Johannes Weiner To: Tejun Heo Cc: Andrew Morton , Michal Hocko , Roman Gushchin , Shakeel Butt , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH 5/8] cgroup: rstat: punt root-level optimization to individual controllers Message-ID: References: <20210205182806.17220-1-hannes@cmpxchg.org> <20210205182806.17220-6-hannes@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 05, 2021 at 10:34:39PM -0500, Tejun Heo wrote: > Hello, > > On Fri, Feb 05, 2021 at 01:28:03PM -0500, Johannes Weiner wrote: > > Current users of the rstat code can source root-level statistics from > > the native counters of their respective subsystem, allowing them to > > forego aggregation at the root level. This optimization is currently > > implemented inside the generic rstat code, which doesn't track the > > root cgroup and doesn't invoke the subsystem flush callbacks on it. > > > > However, the memory controller cannot do this optimization, because > > cgroup1 breaks out memory specifically for the local level, including > > at the root level. In preparation for the memory controller switching > > to rstat, move the optimization from rstat core to the controllers. > > > > Afterwards, rstat will always track the root cgroup for changes and > > invoke the subsystem callbacks on it; and it's up to the subsystem to > > special-case and skip aggregation of the root cgroup if it can source > > this information through other, cheaper means. > > > > The extra cost of tracking the root cgroup is negligible: on stat > > changes, we actually remove a branch that checks for the root. The > > queueing for a flush touches only per-cpu data, and only the first > > stat change since a flush requires a (per-cpu) lock. > > > > Signed-off-by: Johannes Weiner > > Generally looks good to me. > > Acked-by: Tejun Heo Thanks! > A couple nits below. > > > diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c > > index 02ce2058c14b..76725e1cad7f 100644 > > --- a/block/blk-cgroup.c > > +++ b/block/blk-cgroup.c > > @@ -766,6 +766,10 @@ static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu) > > struct blkcg *blkcg = css_to_blkcg(css); > > struct blkcg_gq *blkg; > > > > + /* Root-level stats are sourced from system-wide IO stats */ > > + if (!cgroup_parent(css->cgroup)) > > + return; > > + > > rcu_read_lock(); > > > > hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) { > > @@ -789,6 +793,7 @@ static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu) > > u64_stats_update_end(&blkg->iostat.sync); > > > > /* propagate global delta to parent */ > > + /* XXX: could skip this if parent is root */ > > if (parent) { > > u64_stats_update_begin(&parent->iostat.sync); > > blkg_iostat_set(&delta, &blkg->iostat.cur); > > Might as well update this similar to cgroup_base_stat_flush()? I meant to revisit that, but I'm never 100% confident when it comes to the interaction and lifetime of css, blkcg and blkg_gq. IIUC, the blkg_gq->parent linkage always matches the css parent linkage; it just exists as an optimization for ancestor walks, which would otherwise have to do radix lookups when going through the css. So with the cgroup_parent() check at the beginning of the function making sure we're looking at a non-root group, blkg_gq->parent should also never be NULL and I can do if (paren->parent) directly, right? > > @@ -58,8 +53,16 @@ void cgroup_rstat_updated(struct cgroup *cgrp, int cpu) > > if (rstatc->updated_next) > > break; > > > > + if (!parent) { > > Maybe useful to note that the node is being marked busy but not added to the > non-existent parent. Makes sense, I'll add a comment. > > + rstatc->updated_next = cgrp; > > + break; > > + } > > + > > + prstatc = cgroup_rstat_cpu(parent, cpu); > > rstatc->updated_next = prstatc->updated_children; > > prstatc->updated_children = cgrp; > > + > > + cgrp = parent; > > } > > > > raw_spin_unlock_irqrestore(cpu_lock, flags); > ... > > static void cgroup_base_stat_flush(struct cgroup *cgrp, int cpu) > > { > > - struct cgroup *parent = cgroup_parent(cgrp); > > struct cgroup_rstat_cpu *rstatc = cgroup_rstat_cpu(cgrp, cpu); > > + struct cgroup *parent = cgroup_parent(cgrp); > > Is this chunk intentional? Yeah, it puts the local variable declarations into reverse christmas tree ordering to make them a bit easier to read. It's a while-at-it cleanup, mostly a force of habit. I can drop it if it bothers you. > > struct cgroup_base_stat cur, delta; > > unsigned seq; > > > > + /* Root-level stats are sourced from system-wide CPU stats */ > > + if (!parent) > > + return; > > + > > /* fetch the current per-cpu values */ > > do { > > seq = __u64_stats_fetch_begin(&rstatc->bsync); > > @@ -326,8 +336,8 @@ static void cgroup_base_stat_flush(struct cgroup *cgrp, int cpu) > > cgroup_base_stat_add(&cgrp->bstat, &delta); > > cgroup_base_stat_add(&rstatc->last_bstat, &delta); > > > > - /* propagate global delta to parent */ > > - if (parent) { > > + /* propagate global delta to parent (unless that's root) */ > > + if (cgroup_parent(parent)) { > > Yeah, this makes sense. Can you add a short while-at-it note in the patch > description? Will do. Thanks for the review!