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Message-ID: <ded96eba-8c0c-1822-61b5-de0577b7ebab@redhat.com>
Date: Thu, 22 Apr 2021 12:58:52 -0400
From: Waiman Long <llong@...hat.com>
To: Roman Gushchin <guro@...com>
Cc: Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...nel.org>,
Vladimir Davydov <vdavydov.dev@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
Pekka Enberg <penberg@...nel.org>,
David Rientjes <rientjes@...gle.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
cgroups@...r.kernel.org, linux-mm@...ck.org,
Shakeel Butt <shakeelb@...gle.com>,
Muchun Song <songmuchun@...edance.com>,
Alex Shi <alex.shi@...ux.alibaba.com>,
Chris Down <chris@...isdown.name>,
Yafang Shao <laoar.shao@...il.com>,
Wei Yang <richard.weiyang@...il.com>,
Masayoshi Mizuma <msys.mizuma@...il.com>,
Xing Zhengjun <zhengjun.xing@...ux.intel.com>,
Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH-next v5 2/4] mm/memcg: Cache vmstat data in percpu
memcg_stock_pcp
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@...hat.com>
>> Reviewed-by: Shakeel Butt <shakeelb@...gle.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
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