Messages in this thread | | | From | Waiman Long <> | Subject | Re: [PATCH-next v5 2/4] mm/memcg: Cache vmstat data in percpu memcg_stock_pcp | Date | Thu, 22 Apr 2021 12:58:52 -0400 |
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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
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