Re: [PATCH 2/2] mm: memcontrol: flush percpu slab vmstats on kmem offlining

[Roman Gushchin <guro@fb.com>]

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!