| Message ID | 20230726002904.655377-2-yosryahmed@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] mm: memcg: use rstat for non-hierarchical stats | expand |
On Tue, Jul 25, 2023 at 5:29 PM Yosry Ahmed <yosryahmed@google.com> 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 > 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 > 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 <yosryahmed@google.com> > Acked-by: Johannes Weiner <hannes@cmpxchg.org> > > --- > > v1 -> v2: > - Rewrite the changelog based on the history context provided by > Johannes (Thanks!). > - Fix a subtle bug where updating a local counter would be missed if it > was cancelled out by a pending update from child memcgs. Johannes, I fixed a subtle bug here and I kept your Ack, I wasn't sure what the Ack retention policy should be here. A quick look at the fix would be great. Thanks! > > > --- > include/linux/memcontrol.h | 7 ++-- > mm/memcontrol.c | 67 +++++++++++++++++++++----------------- > mm/workingset.c | 1 + > 3 files changed, 43 insertions(+), 32 deletions(-) > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > index 5818af8eca5a..a9f2861a57a5 100644 > --- a/include/linux/memcontrol.h > +++ b/include/linux/memcontrol.h > @@ -112,6 +112,9 @@ struct lruvec_stats { > /* Aggregated (CPU and subtree) state */ > long state[NR_VM_NODE_STAT_ITEMS]; > > + /* Non-hierarchical (CPU aggregated) state */ > + long state_local[NR_VM_NODE_STAT_ITEMS]; > + > /* Pending child counts during tree propagation */ > long state_pending[NR_VM_NODE_STAT_ITEMS]; > }; > @@ -1020,14 +1023,12 @@ static inline unsigned long lruvec_page_state_local(struct lruvec *lruvec, > { > struct mem_cgroup_per_node *pn; > long x = 0; > - int cpu; > > if (mem_cgroup_disabled()) > return node_page_state(lruvec_pgdat(lruvec), idx); > > pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec); > - for_each_possible_cpu(cpu) > - x += per_cpu(pn->lruvec_stats_percpu->state[idx], cpu); > + x = READ_ONCE(pn->lruvec_stats.state_local[idx]); > #ifdef CONFIG_SMP > if (x < 0) > x = 0; > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index e8ca4bdcb03c..50f8035e998a 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -742,6 +742,10 @@ struct memcg_vmstats { > long state[MEMCG_NR_STAT]; > unsigned long events[NR_MEMCG_EVENTS]; > > + /* Non-hierarchical (CPU aggregated) page state & events */ > + long state_local[MEMCG_NR_STAT]; > + unsigned long events_local[NR_MEMCG_EVENTS]; > + > /* Pending child counts during tree propagation */ > long state_pending[MEMCG_NR_STAT]; > unsigned long events_pending[NR_MEMCG_EVENTS]; > @@ -775,11 +779,8 @@ void __mod_memcg_state(struct mem_cgroup *memcg, int idx, int val) > /* idx can be of type enum memcg_stat_item or node_stat_item. */ > static unsigned long memcg_page_state_local(struct mem_cgroup *memcg, int idx) > { > - long x = 0; > - int cpu; > + long x = READ_ONCE(memcg->vmstats->state_local[idx]); > > - for_each_possible_cpu(cpu) > - x += per_cpu(memcg->vmstats_percpu->state[idx], cpu); > #ifdef CONFIG_SMP > if (x < 0) > x = 0; > @@ -926,16 +927,12 @@ static unsigned long memcg_events(struct mem_cgroup *memcg, int event) > > static unsigned long memcg_events_local(struct mem_cgroup *memcg, int event) > { > - long x = 0; > - int cpu; > int index = memcg_events_index(event); > > if (index < 0) > return 0; > > - for_each_possible_cpu(cpu) > - x += per_cpu(memcg->vmstats_percpu->events[index], cpu); > - return x; > + return READ_ONCE(memcg->vmstats->events_local[index]); > } > > static void mem_cgroup_charge_statistics(struct mem_cgroup *memcg, > @@ -5526,7 +5523,7 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu) > struct mem_cgroup *memcg = mem_cgroup_from_css(css); > struct mem_cgroup *parent = parent_mem_cgroup(memcg); > struct memcg_vmstats_percpu *statc; > - long delta, v; > + long delta, delta_cpu, v; > int i, nid; > > statc = per_cpu_ptr(memcg->vmstats_percpu, cpu); > @@ -5542,19 +5539,23 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu) > memcg->vmstats->state_pending[i] = 0; > > /* Add CPU changes on this level since the last flush */ > + delta_cpu = 0; > v = READ_ONCE(statc->state[i]); > if (v != statc->state_prev[i]) { > - delta += v - statc->state_prev[i]; > + delta_cpu = v - statc->state_prev[i]; > + delta += delta_cpu; > statc->state_prev[i] = v; > } > > - if (!delta) > - continue; > - > /* Aggregate counts on this level and propagate upwards */ > - memcg->vmstats->state[i] += delta; > - if (parent) > - parent->vmstats->state_pending[i] += delta; > + if (delta_cpu) > + memcg->vmstats->state_local[i] += delta_cpu; > + > + if (delta) { > + memcg->vmstats->state[i] += delta; > + if (parent) > + parent->vmstats->state_pending[i] += delta; > + } > } > > for (i = 0; i < NR_MEMCG_EVENTS; i++) { > @@ -5562,18 +5563,22 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu) > if (delta) > memcg->vmstats->events_pending[i] = 0; > > + delta_cpu = 0; > v = READ_ONCE(statc->events[i]); > if (v != statc->events_prev[i]) { > - delta += v - statc->events_prev[i]; > + delta_cpu = v - statc->events_prev[i]; > + delta += delta_cpu; > statc->events_prev[i] = v; > } > > - if (!delta) > - continue; > + if (delta_cpu) > + memcg->vmstats->events_local[i] += delta_cpu; > > - memcg->vmstats->events[i] += delta; > - if (parent) > - parent->vmstats->events_pending[i] += delta; > + if (delta) { > + memcg->vmstats->events[i] += delta; > + if (parent) > + parent->vmstats->events_pending[i] += delta; > + } > } > > for_each_node_state(nid, N_MEMORY) { > @@ -5591,18 +5596,22 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu) > if (delta) > pn->lruvec_stats.state_pending[i] = 0; > > + delta_cpu = 0; > v = READ_ONCE(lstatc->state[i]); > if (v != lstatc->state_prev[i]) { > - delta += v - lstatc->state_prev[i]; > + delta_cpu = v - lstatc->state_prev[i]; > + delta += delta_cpu; > lstatc->state_prev[i] = v; > } > > - if (!delta) > - continue; > + if (delta_cpu) > + pn->lruvec_stats.state_local[i] += delta_cpu; > > - pn->lruvec_stats.state[i] += delta; > - if (ppn) > - ppn->lruvec_stats.state_pending[i] += delta; > + if (delta) { > + pn->lruvec_stats.state[i] += delta; > + if (ppn) > + ppn->lruvec_stats.state_pending[i] += delta; > + } > } > } > } > diff --git a/mm/workingset.c b/mm/workingset.c > index 4686ae363000..da58a26d0d4d 100644 > --- a/mm/workingset.c > +++ b/mm/workingset.c > @@ -664,6 +664,7 @@ static unsigned long count_shadow_nodes(struct shrinker *shrinker, > struct lruvec *lruvec; > int i; > > + mem_cgroup_flush_stats(); > lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid)); > for (pages = 0, i = 0; i < NR_LRU_LISTS; i++) > pages += lruvec_page_state_local(lruvec, > -- > 2.41.0.487.g6d72f3e995-goog >
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 <yosryahmed@google.com> > Acked-by: Johannes Weiner <hannes@cmpxchg.org> Acked-by: Roman Gushchin <roman.gushchin@linux.dev> Thank you!
On Tue, Jul 25, 2023 at 7:15 PM Roman Gushchin <roman.gushchin@linux.dev> wrote: > > 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? Do you mean "we do not maintain global counters"? I think "global" is confusing, because it can be thought of as all cpus or as including the subtree (as opposed to local for non-hierarchical stats). > > 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? Ah yes. I am hoping Andrew can amend this but I am happy to send a v3 as well. > > 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 <yosryahmed@google.com> > > Acked-by: Johannes Weiner <hannes@cmpxchg.org> > > Acked-by: Roman Gushchin <roman.gushchin@linux.dev> Thanks! > > Thank you!
On Tue, Jul 25, 2023 at 05:36:45PM -0700, Yosry Ahmed wrote: > On Tue, Jul 25, 2023 at 5:29 PM Yosry Ahmed <yosryahmed@google.com> wrote: > > - Fix a subtle bug where updating a local counter would be missed if it > > was cancelled out by a pending update from child memcgs. > > > Johannes, I fixed a subtle bug here and I kept your Ack, I wasn't sure > what the Ack retention policy should be here. A quick look at the fix > would be great. Ah, I found it: > > @@ -5542,19 +5539,23 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu) > > memcg->vmstats->state_pending[i] = 0; > > > > /* Add CPU changes on this level since the last flush */ > > + delta_cpu = 0; > > v = READ_ONCE(statc->state[i]); > > if (v != statc->state_prev[i]) { > > - delta += v - statc->state_prev[i]; > > + delta_cpu = v - statc->state_prev[i]; > > + delta += delta_cpu; > > statc->state_prev[i] = v; > > } > > > > - if (!delta) > > - continue; > > - > > /* Aggregate counts on this level and propagate upwards */ > > - memcg->vmstats->state[i] += delta; > > - if (parent) > > - parent->vmstats->state_pending[i] += delta; > > + if (delta_cpu) > > + memcg->vmstats->state_local[i] += delta_cpu; When delta nulls out, but delta_cpu is non-zero... subtle. This fixed version looks good, please keep my ack :)
On Wed, Jul 26, 2023 at 8:13 AM Johannes Weiner <hannes@cmpxchg.org> wrote: > > On Tue, Jul 25, 2023 at 05:36:45PM -0700, Yosry Ahmed wrote: > > On Tue, Jul 25, 2023 at 5:29 PM Yosry Ahmed <yosryahmed@google.com> wrote: > > > - Fix a subtle bug where updating a local counter would be missed if it > > > was cancelled out by a pending update from child memcgs. > > > > > > Johannes, I fixed a subtle bug here and I kept your Ack, I wasn't sure > > what the Ack retention policy should be here. A quick look at the fix > > would be great. > > Ah, I found it: > > > > @@ -5542,19 +5539,23 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu) > > > memcg->vmstats->state_pending[i] = 0; > > > > > > /* Add CPU changes on this level since the last flush */ > > > + delta_cpu = 0; > > > v = READ_ONCE(statc->state[i]); > > > if (v != statc->state_prev[i]) { > > > - delta += v - statc->state_prev[i]; > > > + delta_cpu = v - statc->state_prev[i]; > > > + delta += delta_cpu; > > > statc->state_prev[i] = v; > > > } > > > > > > - if (!delta) > > > - continue; > > > - > > > /* Aggregate counts on this level and propagate upwards */ > > > - memcg->vmstats->state[i] += delta; > > > - if (parent) > > > - parent->vmstats->state_pending[i] += delta; > > > + if (delta_cpu) > > > + memcg->vmstats->state_local[i] += delta_cpu; > > When delta nulls out, but delta_cpu is non-zero... subtle. Yup, led to a few missed updates, was not trivial to track down :) > > This fixed version looks good, please keep my ack :) Thanks!
On Tue, Jul 25, 2023 at 07:20:02PM -0700, Yosry Ahmed wrote: > On Tue, Jul 25, 2023 at 7:15 PM Roman Gushchin <roman.gushchin@linux.dev> wrote: > > > > 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? > > Do you mean "we do not maintain global counters"? I think "global" is > confusing, because it can be thought of as all cpus or as including > the subtree (as opposed to local for non-hierarchical stats). "global" seems fine to me, I don't think it's ambiguous in the direct comparison with per-cpu counts. Alternatively, rephrase the whole thing? Something like: "Non-hierarchical stats are currently not covered by rstat. Their per-cpu counters are summed up on every read, which is expensive."
On Wed, Jul 26, 2023 at 8:20 AM Johannes Weiner <hannes@cmpxchg.org> wrote: > > On Tue, Jul 25, 2023 at 07:20:02PM -0700, Yosry Ahmed wrote: > > On Tue, Jul 25, 2023 at 7:15 PM Roman Gushchin <roman.gushchin@linux.dev> wrote: > > > > > > 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? > > > > Do you mean "we do not maintain global counters"? I think "global" is > > confusing, because it can be thought of as all cpus or as including > > the subtree (as opposed to local for non-hierarchical stats). > > "global" seems fine to me, I don't think it's ambiguous in the direct > comparison with per-cpu counts. > > Alternatively, rephrase the whole thing? Something like: > > "Non-hierarchical stats are currently not covered by rstat. Their > per-cpu counters are summed up on every read, which is expensive." Rephrasing sounds good to me. I will send a v3 with the correct commit log and collected Acks to make Andrew's life easier.
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h index 5818af8eca5a..a9f2861a57a5 100644 --- a/include/linux/memcontrol.h +++ b/include/linux/memcontrol.h @@ -112,6 +112,9 @@ struct lruvec_stats { /* Aggregated (CPU and subtree) state */ long state[NR_VM_NODE_STAT_ITEMS]; + /* Non-hierarchical (CPU aggregated) state */ + long state_local[NR_VM_NODE_STAT_ITEMS]; + /* Pending child counts during tree propagation */ long state_pending[NR_VM_NODE_STAT_ITEMS]; }; @@ -1020,14 +1023,12 @@ static inline unsigned long lruvec_page_state_local(struct lruvec *lruvec, { struct mem_cgroup_per_node *pn; long x = 0; - int cpu; if (mem_cgroup_disabled()) return node_page_state(lruvec_pgdat(lruvec), idx); pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec); - for_each_possible_cpu(cpu) - x += per_cpu(pn->lruvec_stats_percpu->state[idx], cpu); + x = READ_ONCE(pn->lruvec_stats.state_local[idx]); #ifdef CONFIG_SMP if (x < 0) x = 0; diff --git a/mm/memcontrol.c b/mm/memcontrol.c index e8ca4bdcb03c..50f8035e998a 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -742,6 +742,10 @@ struct memcg_vmstats { long state[MEMCG_NR_STAT]; unsigned long events[NR_MEMCG_EVENTS]; + /* Non-hierarchical (CPU aggregated) page state & events */ + long state_local[MEMCG_NR_STAT]; + unsigned long events_local[NR_MEMCG_EVENTS]; + /* Pending child counts during tree propagation */ long state_pending[MEMCG_NR_STAT]; unsigned long events_pending[NR_MEMCG_EVENTS]; @@ -775,11 +779,8 @@ void __mod_memcg_state(struct mem_cgroup *memcg, int idx, int val) /* idx can be of type enum memcg_stat_item or node_stat_item. */ static unsigned long memcg_page_state_local(struct mem_cgroup *memcg, int idx) { - long x = 0; - int cpu; + long x = READ_ONCE(memcg->vmstats->state_local[idx]); - for_each_possible_cpu(cpu) - x += per_cpu(memcg->vmstats_percpu->state[idx], cpu); #ifdef CONFIG_SMP if (x < 0) x = 0; @@ -926,16 +927,12 @@ static unsigned long memcg_events(struct mem_cgroup *memcg, int event) static unsigned long memcg_events_local(struct mem_cgroup *memcg, int event) { - long x = 0; - int cpu; int index = memcg_events_index(event); if (index < 0) return 0; - for_each_possible_cpu(cpu) - x += per_cpu(memcg->vmstats_percpu->events[index], cpu); - return x; + return READ_ONCE(memcg->vmstats->events_local[index]); } static void mem_cgroup_charge_statistics(struct mem_cgroup *memcg, @@ -5526,7 +5523,7 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu) struct mem_cgroup *memcg = mem_cgroup_from_css(css); struct mem_cgroup *parent = parent_mem_cgroup(memcg); struct memcg_vmstats_percpu *statc; - long delta, v; + long delta, delta_cpu, v; int i, nid; statc = per_cpu_ptr(memcg->vmstats_percpu, cpu); @@ -5542,19 +5539,23 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu) memcg->vmstats->state_pending[i] = 0; /* Add CPU changes on this level since the last flush */ + delta_cpu = 0; v = READ_ONCE(statc->state[i]); if (v != statc->state_prev[i]) { - delta += v - statc->state_prev[i]; + delta_cpu = v - statc->state_prev[i]; + delta += delta_cpu; statc->state_prev[i] = v; } - if (!delta) - continue; - /* Aggregate counts on this level and propagate upwards */ - memcg->vmstats->state[i] += delta; - if (parent) - parent->vmstats->state_pending[i] += delta; + if (delta_cpu) + memcg->vmstats->state_local[i] += delta_cpu; + + if (delta) { + memcg->vmstats->state[i] += delta; + if (parent) + parent->vmstats->state_pending[i] += delta; + } } for (i = 0; i < NR_MEMCG_EVENTS; i++) { @@ -5562,18 +5563,22 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu) if (delta) memcg->vmstats->events_pending[i] = 0; + delta_cpu = 0; v = READ_ONCE(statc->events[i]); if (v != statc->events_prev[i]) { - delta += v - statc->events_prev[i]; + delta_cpu = v - statc->events_prev[i]; + delta += delta_cpu; statc->events_prev[i] = v; } - if (!delta) - continue; + if (delta_cpu) + memcg->vmstats->events_local[i] += delta_cpu; - memcg->vmstats->events[i] += delta; - if (parent) - parent->vmstats->events_pending[i] += delta; + if (delta) { + memcg->vmstats->events[i] += delta; + if (parent) + parent->vmstats->events_pending[i] += delta; + } } for_each_node_state(nid, N_MEMORY) { @@ -5591,18 +5596,22 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu) if (delta) pn->lruvec_stats.state_pending[i] = 0; + delta_cpu = 0; v = READ_ONCE(lstatc->state[i]); if (v != lstatc->state_prev[i]) { - delta += v - lstatc->state_prev[i]; + delta_cpu = v - lstatc->state_prev[i]; + delta += delta_cpu; lstatc->state_prev[i] = v; } - if (!delta) - continue; + if (delta_cpu) + pn->lruvec_stats.state_local[i] += delta_cpu; - pn->lruvec_stats.state[i] += delta; - if (ppn) - ppn->lruvec_stats.state_pending[i] += delta; + if (delta) { + pn->lruvec_stats.state[i] += delta; + if (ppn) + ppn->lruvec_stats.state_pending[i] += delta; + } } } } diff --git a/mm/workingset.c b/mm/workingset.c index 4686ae363000..da58a26d0d4d 100644 --- a/mm/workingset.c +++ b/mm/workingset.c @@ -664,6 +664,7 @@ static unsigned long count_shadow_nodes(struct shrinker *shrinker, struct lruvec *lruvec; int i; + mem_cgroup_flush_stats(); lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid)); for (pages = 0, i = 0; i < NR_LRU_LISTS; i++) pages += lruvec_page_state_local(lruvec,
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 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 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 <yosryahmed@google.com> Acked-by: Johannes Weiner <hannes@cmpxchg.org> --- v1 -> v2: - Rewrite the changelog based on the history context provided by Johannes (Thanks!). - Fix a subtle bug where updating a local counter would be missed if it was cancelled out by a pending update from child memcgs. --- include/linux/memcontrol.h | 7 ++-- mm/memcontrol.c | 67 +++++++++++++++++++++----------------- mm/workingset.c | 1 + 3 files changed, 43 insertions(+), 32 deletions(-)