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

[Waiman Long]

On 4/22/21 9:56 PM, Roman Gushchin wrote:
> On Thu, Apr 22, 2021 at 12:58:52PM -0400, Waiman Long wrote:
> > 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.
> Hm, why? We wouldn't need to flush the stock if the release happens
> on some other cpu not matching the current pgdat.

I had actually experimented with just caching vmstat data for the local 
node only. It turned out the hit rate was a bit lower. That is why I 
keep the current approach and I need to do further investigation on a 
better approach.

Cheers,
Longman