| Message ID | 20190812222911.2364802-3-guro@fb.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | flush percpu vmstats | expand |
On Mon 12-08-19 15:29:11, Roman Gushchin wrote: > I've noticed that the "slab" value in memory.stat is sometimes 0, > even if some children memory cgroups have a non-zero "slab" value. > The following investigation showed that this is the result > of the kmem_cache reparenting in combination with the per-cpu > batching of slab vmstats. > > At the offlining some vmstat value may leave in the percpu cache, > not being propagated upwards by the cgroup hierarchy. It means > that stats on ancestor levels are lower than actual. Later when > slab pages are released, the precise number of pages is substracted > on the parent level, making the value negative. We don't show negative > values, 0 is printed instead. So the difference with other counters is that slab ones are reparented and that's why we have treat them specially? I guess that is what the comment in the code suggest but being explicit in the changelog would be nice. [...] > -static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg) > +static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg, bool slab_only) > { > unsigned long stat[MEMCG_NR_STAT]; > struct mem_cgroup *mi; > int node, cpu, i; > + int min_idx, max_idx; > > - for (i = 0; i < MEMCG_NR_STAT; i++) > + if (slab_only) { > + min_idx = NR_SLAB_RECLAIMABLE; > + max_idx = NR_SLAB_UNRECLAIMABLE; > + } else { > + min_idx = 0; > + max_idx = MEMCG_NR_STAT; > + } This is just ugly has hell! I really detest how this implicitly makes counters value very special without any note in the node_stat_item definition. Is it such a big deal to have a per counter flush and do the loop over all counters resp. specific counters around it so much worse? This should be really a slow path to safe few instructions or cache misses, no?
On Wed, Aug 14, 2019 at 01:32:42PM +0200, Michal Hocko wrote: > On Mon 12-08-19 15:29:11, Roman Gushchin wrote: > > I've noticed that the "slab" value in memory.stat is sometimes 0, > > even if some children memory cgroups have a non-zero "slab" value. > > The following investigation showed that this is the result > > of the kmem_cache reparenting in combination with the per-cpu > > batching of slab vmstats. > > > > At the offlining some vmstat value may leave in the percpu cache, > > not being propagated upwards by the cgroup hierarchy. It means > > that stats on ancestor levels are lower than actual. Later when > > slab pages are released, the precise number of pages is substracted > > on the parent level, making the value negative. We don't show negative > > values, 0 is printed instead. > > So the difference with other counters is that slab ones are reparented > and that's why we have treat them specially? I guess that is what the > comment in the code suggest but being explicit in the changelog would be > nice. Right. And I believe the list can be extended further. Objects which are often outliving the origin memory cgroup (e.g. pagecache pages) are pinning dead cgroups, so it will be cool to reparent them all. > > [...] > > -static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg) > > +static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg, bool slab_only) > > { > > unsigned long stat[MEMCG_NR_STAT]; > > struct mem_cgroup *mi; > > int node, cpu, i; > > + int min_idx, max_idx; > > > > - for (i = 0; i < MEMCG_NR_STAT; i++) > > + if (slab_only) { > > + min_idx = NR_SLAB_RECLAIMABLE; > > + max_idx = NR_SLAB_UNRECLAIMABLE; > > + } else { > > + min_idx = 0; > > + max_idx = MEMCG_NR_STAT; > > + } > > This is just ugly has hell! I really detest how this implicitly makes > counters value very special without any note in the node_stat_item > definition. Is it such a big deal to have a per counter flush and do > the loop over all counters resp. specific counters around it so much > worse? This should be really a slow path to safe few instructions or > cache misses, no? I believe that it is a big deal, because it's NR_VMSTAT_ITEMS * all memory cgroups * online cpus * numa nodes. If the goal is to merge it with cpu hotplug code, I'd think about passing cpumask to it, and do the opposite. Also I'm not sure I understand why reordering loops will make it less ugly. But you're right, a comment nearby NR_SLAB_(UN)RECLAIMABLE definition is totaly worth it. How about something like: diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 8b5f758942a2..231bcbe5dcc6 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -215,8 +215,9 @@ enum node_stat_item { NR_INACTIVE_FILE, /* " " " " " */ NR_ACTIVE_FILE, /* " " " " " */ NR_UNEVICTABLE, /* " " " " " */ - NR_SLAB_RECLAIMABLE, - NR_SLAB_UNRECLAIMABLE, + NR_SLAB_RECLAIMABLE, /* Please, do not reorder this item */ + NR_SLAB_UNRECLAIMABLE, /* and this one without looking at + * memcg_flush_percpu_vmstats() first. */ NR_ISOLATED_ANON, /* Temporary isolated pages from anon lru */ NR_ISOLATED_FILE, /* Temporary isolated pages from file lru */ WORKINGSET_NODES, -- Thanks!
On Wed 14-08-19 21:54:12, Roman Gushchin wrote: > On Wed, Aug 14, 2019 at 01:32:42PM +0200, Michal Hocko wrote: > > On Mon 12-08-19 15:29:11, Roman Gushchin wrote: > > > I've noticed that the "slab" value in memory.stat is sometimes 0, > > > even if some children memory cgroups have a non-zero "slab" value. > > > The following investigation showed that this is the result > > > of the kmem_cache reparenting in combination with the per-cpu > > > batching of slab vmstats. > > > > > > At the offlining some vmstat value may leave in the percpu cache, > > > not being propagated upwards by the cgroup hierarchy. It means > > > that stats on ancestor levels are lower than actual. Later when > > > slab pages are released, the precise number of pages is substracted > > > on the parent level, making the value negative. We don't show negative > > > values, 0 is printed instead. > > > > So the difference with other counters is that slab ones are reparented > > and that's why we have treat them specially? I guess that is what the > > comment in the code suggest but being explicit in the changelog would be > > nice. > > Right. And I believe the list can be extended further. Objects which > are often outliving the origin memory cgroup (e.g. pagecache pages) > are pinning dead cgroups, so it will be cool to reparent them all. > > > > > [...] > > > -static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg) > > > +static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg, bool slab_only) > > > { > > > unsigned long stat[MEMCG_NR_STAT]; > > > struct mem_cgroup *mi; > > > int node, cpu, i; > > > + int min_idx, max_idx; > > > > > > - for (i = 0; i < MEMCG_NR_STAT; i++) > > > + if (slab_only) { > > > + min_idx = NR_SLAB_RECLAIMABLE; > > > + max_idx = NR_SLAB_UNRECLAIMABLE; > > > + } else { > > > + min_idx = 0; > > > + max_idx = MEMCG_NR_STAT; > > > + } > > > > This is just ugly has hell! I really detest how this implicitly makes > > counters value very special without any note in the node_stat_item > > definition. Is it such a big deal to have a per counter flush and do > > the loop over all counters resp. specific counters around it so much > > worse? This should be really a slow path to safe few instructions or > > cache misses, no? > > I believe that it is a big deal, because it's > NR_VMSTAT_ITEMS * all memory cgroups * online cpus * numa nodes. I am not sure I follow. I just meant to remove all for (i = 0; i < MEMCG_NR_STAT; i++) from flushing and do that loop around the flushing function. That would mean that the NR_SLAB_$FOO wouldn't have to play tricks and simply call the flushing for the two counters. > If the goal is to merge it with cpu hotplug code, I'd think about passing > cpumask to it, and do the opposite. Also I'm not sure I understand > why reordering loops will make it less ugly. And adding a cpu/nodemasks would just work with that as well, right. > > But you're right, a comment nearby NR_SLAB_(UN)RECLAIMABLE definition > is totaly worth it. How about something like: > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > index 8b5f758942a2..231bcbe5dcc6 100644 > --- a/include/linux/mmzone.h > +++ b/include/linux/mmzone.h > @@ -215,8 +215,9 @@ enum node_stat_item { > NR_INACTIVE_FILE, /* " " " " " */ > NR_ACTIVE_FILE, /* " " " " " */ > NR_UNEVICTABLE, /* " " " " " */ > - NR_SLAB_RECLAIMABLE, > - NR_SLAB_UNRECLAIMABLE, > + NR_SLAB_RECLAIMABLE, /* Please, do not reorder this item */ > + NR_SLAB_UNRECLAIMABLE, /* and this one without looking at > + * memcg_flush_percpu_vmstats() first. */ > NR_ISOLATED_ANON, /* Temporary isolated pages from anon lru */ > NR_ISOLATED_FILE, /* Temporary isolated pages from file lru */ > WORKINGSET_NODES, Thanks, that is an improvement.
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 348f685ab94b..6d2427abcc0c 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -3412,37 +3412,49 @@ static int memcg_online_kmem(struct mem_cgroup *memcg) return 0; } -static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg) +static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg, bool slab_only) { unsigned long stat[MEMCG_NR_STAT]; struct mem_cgroup *mi; int node, cpu, i; + int min_idx, max_idx; - for (i = 0; i < MEMCG_NR_STAT; i++) + if (slab_only) { + min_idx = NR_SLAB_RECLAIMABLE; + max_idx = NR_SLAB_UNRECLAIMABLE; + } else { + min_idx = 0; + max_idx = MEMCG_NR_STAT; + } + + for (i = min_idx; i < max_idx; i++) stat[i] = 0; for_each_online_cpu(cpu) - for (i = 0; i < MEMCG_NR_STAT; i++) + for (i = min_idx; i < max_idx; i++) stat[i] += raw_cpu_read(memcg->vmstats_percpu->stat[i]); for (mi = memcg; mi; mi = parent_mem_cgroup(mi)) - for (i = 0; i < MEMCG_NR_STAT; i++) + for (i = min_idx; i < max_idx; i++) atomic_long_add(stat[i], &mi->vmstats[i]); + if (!slab_only) + max_idx = NR_VM_NODE_STAT_ITEMS; + for_each_node(node) { struct mem_cgroup_per_node *pn = memcg->nodeinfo[node]; struct mem_cgroup_per_node *pi; - for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++) + for (i = min_idx; i < max_idx; i++) stat[i] = 0; for_each_online_cpu(cpu) - for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++) + for (i = min_idx; i < max_idx; i++) stat[i] += raw_cpu_read( pn->lruvec_stat_cpu->count[i]); for (pi = pn; pi; pi = parent_nodeinfo(pi, node)) - for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++) + for (i = min_idx; i < max_idx; i++) atomic_long_add(stat[i], &pi->lruvec_stat[i]); } } @@ -3467,7 +3479,14 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg) if (!parent) parent = root_mem_cgroup; + /* + * Deactivate and reparent kmem_caches. Then flush percpu + * slab statistics to have precise values at the parent and + * all ancestor levels. It's required to keep slab stats + * accurate after the reparenting of kmem_caches. + */ memcg_deactivate_kmem_caches(memcg, parent); + memcg_flush_percpu_vmstats(memcg, true); kmemcg_id = memcg->kmemcg_id; BUG_ON(kmemcg_id < 0); @@ -4844,7 +4863,7 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg) * Flush percpu vmstats to guarantee the value correctness * on parent's and all ancestor levels. */ - memcg_flush_percpu_vmstats(memcg); + memcg_flush_percpu_vmstats(memcg, false); for_each_node(node) free_mem_cgroup_per_node_info(memcg, node); free_percpu(memcg->vmstats_percpu);
I've noticed that the "slab" value in memory.stat is sometimes 0, even if some children memory cgroups have a non-zero "slab" value. The following investigation showed that this is the result of the kmem_cache reparenting in combination with the per-cpu batching of slab vmstats. At the offlining some vmstat value may leave in the percpu cache, not being propagated upwards by the cgroup hierarchy. It means that stats on ancestor levels are lower than actual. Later when slab pages are released, the precise number of pages is substracted on the parent level, making the value negative. We don't show negative values, 0 is printed instead. To fix this issue, let's flush percpu slab memcg and lruvec stats on memcg offlining. This guarantees that numbers on all ancestor levels are accurate and match the actual number of outstanding slab pages. Fixes: fb2f2b0adb98 ("mm: memcg/slab: reparent memcg kmem_caches on cgroup removal") Signed-off-by: Roman Gushchin <guro@fb.com> Cc: Johannes Weiner <hannes@cmpxchg.org> --- mm/memcontrol.c | 35 +++++++++++++++++++++++++++-------- 1 file changed, 27 insertions(+), 8 deletions(-)