Re: [PATCH v4 2/5] mm/memcg: Cache vmstat data in percpu memcg_stock_pcp

From: Waiman Long
Date: Mon Apr 19 2021 - 19:42:14 EST

On 4/19/21 12:38 PM, Johannes Weiner wrote:
On Sun, Apr 18, 2021 at 08:00:29PM -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 | 64 ++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 61 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index dc9032f28f2e..693453f95d99 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2213,7 +2213,10 @@ struct memcg_stock_pcp {
#ifdef CONFIG_MEMCG_KMEM
struct obj_cgroup *cached_objcg;
+ struct pglist_data *cached_pgdat;
unsigned int nr_bytes;
+ int vmstat_idx;
+ int vmstat_bytes;
#endif
struct work_struct work;
@@ -3150,8 +3153,9 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
css_put(&memcg->css);
}
-void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
- enum node_stat_item idx, int nr)
+static inline void __mod_objcg_state(struct obj_cgroup *objcg,
+ struct pglist_data *pgdat,
+ enum node_stat_item idx, int nr)
This naming is dangerous, as the __mod_foo naming scheme we use
everywhere else suggests it's the same function as mod_foo() just with
preemption/irqs disabled.

I will change its name to, say, mod_objcg_mlstate() to indicate that it is something different. Actually, it is hard to come up with a good name which is not too long.


@@ -3159,10 +3163,53 @@ 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();
}
+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;
+
+ 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) {
+ /* Output the current data as is */
When you get here with the wrong objcg and hit the cold path, it's
usually immediately followed by an uncharge -> refill_obj_stock() that
will then flush and reset cached_objcg.

Instead of doing two cold paths, why not flush the old objcg right
away and set the new so that refill_obj_stock() can use the fast path?

That is a good idea. Will do that.



+ } else if (!stock->vmstat_bytes) {
+ /* Save the current data */
+ stock->vmstat_bytes = nr;
+ stock->vmstat_idx = idx;
+ stock->cached_pgdat = pgdat;
+ nr = 0;
+ } else if ((stock->cached_pgdat != pgdat) ||
+ (stock->vmstat_idx != idx)) {
+ /* Output the cached data & save the current data */
+ swap(nr, stock->vmstat_bytes);
+ swap(idx, stock->vmstat_idx);
+ swap(pgdat, stock->cached_pgdat);
Is this optimization worth doing?

You later split vmstat_bytes and idx doesn't change anymore.

I am going to merge patch 2 and patch 4 to avoid the confusion.



How often does the pgdat change? This is a per-cpu cache after all,
and the numa node a given cpu allocates from tends to not change that
often. Even with interleaving mode, which I think is pretty rare, the
interleaving happens at the slab/page level, not the object level, and
the cache isn't bigger than a page anyway.

The testing done on a 2-socket system indicated that pgdat changes roughly 10-20% of time. So it does happen, especially on the kfree() path, I think. I have tried to cached vmstat update for those on the local node only, but I got more misses with that. So I am just going to change pgdat and flush out existing data for now.



+ } else {
+ stock->vmstat_bytes += nr;
+ if (abs(stock->vmstat_bytes) > PAGE_SIZE) {
+ nr = stock->vmstat_bytes;
+ stock->vmstat_bytes = 0;
+ } else {
+ nr = 0;
+ }
..and this is the regular overflow handling done by the objcg and
memcg charge stock as well.

How about this?

if (stock->cached_objcg != objcg ||
stock->cached_pgdat != pgdat ||
stock->vmstat_idx != idx) {
drain_obj_stock(stock);
obj_cgroup_get(objcg);
stock->cached_objcg = objcg;
stock->nr_bytes = atomic_xchg(&objcg->nr_charged_bytes, 0);
stock->vmstat_idx = idx;
}
stock->vmstat_bytes += nr_bytes;

if (abs(stock->vmstat_bytes > PAGE_SIZE))
drain_obj_stock(stock);

(Maybe we could be clever, here since the charge and stat caches are
the same size: don't flush an oversized charge cache from
refill_obj_stock in the charge path, but leave it to the
mod_objcg_state() that follows; likewise don't flush an undersized
vmstat stock from mod_objcg_state() in the uncharge path, but leave it
to the refill_obj_stock() that follows. Could get a bit complicated...)

If you look at patch 5, I am trying to avoid doing drain_obj_stock() unless the objcg change. I am going to do the same here.

Cheers,
Longman