From: Johannes Weiner <hannes@xxxxxxxxxxx> Subject: cgroup: rstat: punt root-level optimization to individual controllers 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. This is the case for the io controller and the cgroup base stats. In their respective flush callbacks, check whether the parent is the root cgroup, and if so, skip the unnecessary upward propagation. 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. Link: https://lkml.kernel.org/r/20210209163304.77088-6-hannes@xxxxxxxxxxx Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> Acked-by: Tejun Heo <tj@xxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: Michal Koutný <mkoutny@xxxxxxxx> Cc: Roman Gushchin <guro@xxxxxx> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- block/blk-cgroup.c | 17 +++++++---- kernel/cgroup/rstat.c | 59 +++++++++++++++++++++++----------------- 2 files changed, 46 insertions(+), 30 deletions(-) --- a/block/blk-cgroup.c~cgroup-rstat-punt-root-level-optimization-to-individual-controllers +++ a/block/blk-cgroup.c @@ -764,6 +764,10 @@ static void blkcg_rstat_flush(struct cgr 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) { @@ -786,8 +790,8 @@ static void blkcg_rstat_flush(struct cgr blkg_iostat_add(&bisc->last, &delta); u64_stats_update_end(&blkg->iostat.sync); - /* propagate global delta to parent */ - if (parent) { + /* propagate global delta to parent (unless that's root) */ + if (parent && parent->parent) { u64_stats_update_begin(&parent->iostat.sync); blkg_iostat_set(&delta, &blkg->iostat.cur); blkg_iostat_sub(&delta, &blkg->iostat.last); @@ -801,10 +805,11 @@ static void blkcg_rstat_flush(struct cgr } /* - * The rstat algorithms intentionally don't handle the root cgroup to avoid - * incurring overhead when no cgroups are defined. For that reason, - * cgroup_rstat_flush in blkcg_print_stat does not actually fill out the - * iostat in the root cgroup's blkcg_gq. + * We source root cgroup stats from the system-wide stats to avoid + * tracking the same information twice and incurring overhead when no + * cgroups are defined. For that reason, cgroup_rstat_flush in + * blkcg_print_stat does not actually fill out the iostat in the root + * cgroup's blkcg_gq. * * However, we would like to re-use the printing code between the root and * non-root cgroups to the extent possible. For that reason, we simulate --- a/kernel/cgroup/rstat.c~cgroup-rstat-punt-root-level-optimization-to-individual-controllers +++ a/kernel/cgroup/rstat.c @@ -25,13 +25,8 @@ static struct cgroup_rstat_cpu *cgroup_r void cgroup_rstat_updated(struct cgroup *cgrp, int cpu) { raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu); - struct cgroup *parent; unsigned long flags; - /* nothing to do for root */ - if (!cgroup_parent(cgrp)) - return; - /* * Speculative already-on-list test. This may race leading to * temporary inaccuracies, which is fine. @@ -46,10 +41,10 @@ void cgroup_rstat_updated(struct cgroup raw_spin_lock_irqsave(cpu_lock, flags); /* put @cgrp and all ancestors on the corresponding updated lists */ - for (parent = cgroup_parent(cgrp); parent; - cgrp = parent, parent = cgroup_parent(cgrp)) { + while (true) { struct cgroup_rstat_cpu *rstatc = cgroup_rstat_cpu(cgrp, cpu); - struct cgroup_rstat_cpu *prstatc = cgroup_rstat_cpu(parent, cpu); + struct cgroup *parent = cgroup_parent(cgrp); + struct cgroup_rstat_cpu *prstatc; /* * Both additions and removals are bottom-up. If a cgroup @@ -58,8 +53,17 @@ void cgroup_rstat_updated(struct cgroup if (rstatc->updated_next) break; + /* Root has no parent to link it to, but mark it busy */ + if (!parent) { + 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); @@ -113,23 +117,26 @@ static struct cgroup *cgroup_rstat_cpu_p */ if (rstatc->updated_next) { struct cgroup *parent = cgroup_parent(pos); - struct cgroup_rstat_cpu *prstatc = cgroup_rstat_cpu(parent, cpu); - struct cgroup_rstat_cpu *nrstatc; - struct cgroup **nextp; - - nextp = &prstatc->updated_children; - while (true) { - nrstatc = cgroup_rstat_cpu(*nextp, cpu); - if (*nextp == pos) - break; - WARN_ON_ONCE(*nextp == parent); - nextp = &nrstatc->updated_next; + if (parent) { + struct cgroup_rstat_cpu *prstatc; + struct cgroup **nextp; + + prstatc = cgroup_rstat_cpu(parent, cpu); + nextp = &prstatc->updated_children; + while (true) { + struct cgroup_rstat_cpu *nrstatc; + + nrstatc = cgroup_rstat_cpu(*nextp, cpu); + if (*nextp == pos) + break; + WARN_ON_ONCE(*nextp == parent); + nextp = &nrstatc->updated_next; + } + *nextp = rstatc->updated_next; } - *nextp = rstatc->updated_next; rstatc->updated_next = NULL; - return pos; } @@ -309,11 +316,15 @@ static void cgroup_base_stat_sub(struct 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); 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 +337,8 @@ static void cgroup_base_stat_flush(struc 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)) { delta = cgrp->bstat; cgroup_base_stat_sub(&delta, &cgrp->last_bstat); cgroup_base_stat_add(&parent->bstat, &delta); _
