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. 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
