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

[Waiman Long <llong@redhat.com>]

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@redhat.com>
>> Reviewed-by: Shakeel Butt <shakeelb@google.com>
>> ---
>>   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