Re: [PATCH-next v5 2/4] mm/memcg: Cache vmstat data in percpu memcg_stock_pcp

[Waiman Long]

On 4/21/21 7:28 PM, Roman Gushchin wrote:
> On Tue, Apr 20, 2021 at 03:29:05PM -0400, Waiman Long wrote:
> > Before the new slab memory controller with per object byte charging,
> > charging and vmstat data update happen only when new slab pages are
> > allocated or freed. Now they are done with every kmem_cache_alloc()
> > and kmem_cache_free(). This causes additional overhead for workloads
> > that generate a lot of alloc and free calls.
> >
> > The memcg_stock_pcp is used to cache byte charge for a specific
> > obj_cgroup to reduce that overhead. To further reducing it, this patch
> > makes the vmstat data cached in the memcg_stock_pcp structure as well
> > until it accumulates a page size worth of update or when other cached
> > data change. Caching the vmstat data in the per-cpu stock eliminates two
> > writes to non-hot cachelines for memcg specific as well as memcg-lruvecs
> > specific vmstat data by a write to a hot local stock cacheline.
> >
> > On a 2-socket Cascade Lake server with instrumentation enabled and this
> > patch applied, it was found that about 20% (634400 out of 3243830)
> > of the time when mod_objcg_state() is called leads to an actual call
> > to __mod_objcg_state() after initial boot. When doing parallel kernel
> > build, the figure was about 17% (24329265 out of 142512465). So caching
> > the vmstat data reduces the number of calls to __mod_objcg_state()
> > by more than 80%.
> >
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > ---
> >   mm/memcontrol.c | 86 +++++++++++++++++++++++++++++++++++++++++++++++--
> >   1 file changed, 83 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 7cd7187a017c..292b4783b1a7 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -782,8 +782,9 @@ void __mod_lruvec_kmem_state(void *p, enum node_stat_item idx, int val)
> >   	rcu_read_unlock();
> >   }
> >   
> > -void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
> > -		     enum node_stat_item idx, int nr)
> > +static inline void mod_objcg_mlstate(struct obj_cgroup *objcg,
> > +				     struct pglist_data *pgdat,
> > +				     enum node_stat_item idx, int nr)
> >   {
> >   	struct mem_cgroup *memcg;
> >   	struct lruvec *lruvec;
> > @@ -791,7 +792,7 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
> >   	rcu_read_lock();
> >   	memcg = obj_cgroup_memcg(objcg);
> >   	lruvec = mem_cgroup_lruvec(memcg, pgdat);
> > -	mod_memcg_lruvec_state(lruvec, idx, nr);
> > +	__mod_memcg_lruvec_state(lruvec, idx, nr);
> >   	rcu_read_unlock();
> >   }
> >   
> > @@ -2059,7 +2060,10 @@ struct memcg_stock_pcp {
> >   
> >   #ifdef CONFIG_MEMCG_KMEM
> >   	struct obj_cgroup *cached_objcg;
> > +	struct pglist_data *cached_pgdat;
> I wonder if we want to have per-node counters instead?
> That would complicate the initialization of pcp stocks a bit,
> but might shave off some additional cpu time.
> But we can do it later too.
A per node counter will certainly complicate the code and reduce the 
performance benefit too. I got a pretty good hit rate of 80%+ with the 
current code on a 2-socket system. The hit rate will probably drop when 
there are more nodes. I will do some more investigation, but it will not 
be for this patchset.


> >   	unsigned int nr_bytes;
> > +	int nr_slab_reclaimable_b;
> > +	int nr_slab_unreclaimable_b;
> >   #endif
> >   
> >   	struct work_struct work;
> > @@ -3008,6 +3012,63 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
> >   	obj_cgroup_put(objcg);
> >   }
> >   
> > +void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
> > +		     enum node_stat_item idx, int nr)
> > +{
> > +	struct memcg_stock_pcp *stock;
> > +	unsigned long flags;
> > +	int *bytes;
> > +
> > +	local_irq_save(flags);
> > +	stock = this_cpu_ptr(&memcg_stock);
> > +
> > +	/*
> > +	 * Save vmstat data in stock and skip vmstat array update unless
> > +	 * accumulating over a page of vmstat data or when pgdat or idx
> > +	 * changes.
> > +	 */
> > +	if (stock->cached_objcg != objcg) {
> > +		drain_obj_stock(stock);
> > +		obj_cgroup_get(objcg);
> > +		stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes)
> > +				? atomic_xchg(&objcg->nr_charged_bytes, 0) : 0;
> > +		stock->cached_objcg = objcg;
> > +		stock->cached_pgdat = pgdat;
> > +	} else if (stock->cached_pgdat != pgdat) {
> > +		/* Flush the existing cached vmstat data */
> > +		if (stock->nr_slab_reclaimable_b) {
> > +			mod_objcg_mlstate(objcg, pgdat, NR_SLAB_RECLAIMABLE_B,
> > +					  stock->nr_slab_reclaimable_b);
> > +			stock->nr_slab_reclaimable_b = 0;
> > +		}
> > +		if (stock->nr_slab_unreclaimable_b) {
> > +			mod_objcg_mlstate(objcg, pgdat, NR_SLAB_UNRECLAIMABLE_B,
> > +					  stock->nr_slab_unreclaimable_b);
> > +			stock->nr_slab_unreclaimable_b = 0;
> > +		}
> > +		stock->cached_pgdat = pgdat;
> > +	}
> > +
> > +	bytes = (idx == NR_SLAB_RECLAIMABLE_B) ? &stock->nr_slab_reclaimable_b
> > +					       : &stock->nr_slab_unreclaimable_b;
> > +	if (!*bytes) {
> > +		*bytes = nr;
> > +		nr = 0;
> > +	} else {
> > +		*bytes += nr;
> > +		if (abs(*bytes) > PAGE_SIZE) {
> > +			nr = *bytes;
> > +			*bytes = 0;
> > +		} else {
> > +			nr = 0;
> > +		}
> > +	}
> This part is a little bit hard to follow, how about something like this
> (completely untested):
>
> {
> 	stocked = (idx == NR_SLAB_RECLAIMABLE_B) ? &stock->nr_slab_reclaimable_b
> 		: &stock->nr_slab_unreclaimable_b;
> 	if (abs(*stocked + nr) > PAGE_SIZE) {
> 		nr += *stocked;
> 		*stocked = 0;
> 	} else {
> 		*stocked += nr;
> 		nr = 0;
> 	}
> }

That was done purposely to make sure that large object (>= 4k) will also 
be cached once before flushing it out. I should have been more clear 
about that by adding a comment about it. vmstat data isn't as critical 
as memory charge and so I am allowing it to cache more than 4k in this case.

Cheers,
Longman