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

[Roman Gushchin <guro@fb.com>]

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.

>  	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;
	}
}



> +	if (nr)
> +		mod_objcg_mlstate(objcg, pgdat, idx, nr);
> +
> +	local_irq_restore(flags);
> +}
> +
>  static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
>  {
>  	struct memcg_stock_pcp *stock;
> @@ -3055,6 +3116,25 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
>  		stock->nr_bytes = 0;
>  	}
>  
> +	/*
> +	 * Flush the vmstat data in current stock
> +	 */
> +	if (stock->nr_slab_reclaimable_b || stock->nr_slab_unreclaimable_b) {
> +		if (stock->nr_slab_reclaimable_b) {
> +			mod_objcg_mlstate(old, stock->cached_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(old, stock->cached_pgdat,
> +					  NR_SLAB_UNRECLAIMABLE_B,
> +					  stock->nr_slab_unreclaimable_b);
> +			stock->nr_slab_unreclaimable_b = 0;
> +		}
> +		stock->cached_pgdat = NULL;
> +	}
> +
>  	obj_cgroup_put(old);
>  	stock->cached_objcg = NULL;
>  }
> -- 
> 2.18.1
>