On Mon, Jun 27, 2022 at 6:48 PM Shakeel Butt [off-list ref] wrote:

On Mon, Jun 27, 2022 at 9:26 AM Eric Dumazet [off-list ref] wrote:

quoted

[...]

quoted

quoted

I simply did the following and got much better results.

But I am not sure if updates to ->usage are really needed that often...

I suspect we need to improve the per-cpu memcg stock usage here. Were
the updates mostly from uncharge path or charge path or that's
irrelevant?

I wonder if the cache is always used...

stock = this_cpu_ptr(&memcg_stock);
if (memcg == stock->cached && stock->nr_pages >= nr_pages) {

Apparently the per-cpu cache is only used for one memcg at a time ?

Not sure how this would scale to hosts with dozens of memcgs.

Maybe we could add some metrics to have an idea of the cache hit/miss ratio :/

I think doing full drain (i.e. drain_stock()) within __refill_stock()
when the local cache is larger than MEMCG_CHARGE_BATCH is not best.
Rather we should always keep at least MEMCG_CHARGE_BATCH for such
scenarios.

quoted

diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
index 679591301994d316062f92b275efa2459a8349c9..e267be4ba849760117d9fd041e22c2a44658ab36

100644

--- a/include/linux/page_counter.h
+++ b/include/linux/page_counter.h

@@ -3,12 +3,15 @@
 #define _LINUX_PAGE_COUNTER_H

 #include <linux/atomic.h>
+#include <linux/cache.h>
 #include <linux/kernel.h>
 #include <asm/page.h>

 struct page_counter {
-       atomic_long_t usage;
-       unsigned long min;
+       /* contended cache line. */
+       atomic_long_t usage ____cacheline_aligned_in_smp;
+
+       unsigned long min ____cacheline_aligned_in_smp;

Do we need to align 'min' too?

Probably if there is a hierarchy ...

propagate_protected_usage() seems to have potential high cost.

quoted

        unsigned long low;
        unsigned long high;
        unsigned long max;

@@ -27,12 +30,6 @@ struct page_counter {
        unsigned long watermark;
        unsigned long failcnt;

-       /*
-        * 'parent' is placed here to be far from 'usage' to reduce
-        * cache false sharing, as 'usage' is written mostly while
-        * parent is frequently read for cgroup's hierarchical
-        * counting nature.
-        */
        struct page_counter *parent;
 };