Re: [PATCH v4 10/14] memcg: use static branches when code not inuse

From: Kamezawa Hiroyuki
Date: Tue Oct 16 2012 - 04:48:53 EST

(2012/10/12 16:47), Glauber Costa wrote:
On 10/11/2012 05:40 PM, Michal Hocko wrote:
On Mon 08-10-12 14:06:16, Glauber Costa wrote:
We can use static branches to patch the code in or out when not used.

Because the _ACTIVE bit on kmem_accounted is only set after the
increment is done, we guarantee that the root memcg will always be
selected for kmem charges until all call sites are patched (see
memcg_kmem_enabled). This guarantees that no mischarges are applied.

static branch decrement happens when the last reference count from the
kmem accounting in memcg dies. This will only happen when the charges
drop down to 0.

When that happen, we need to disable the static branch only on those
memcgs that enabled it. To achieve this, we would be forced to
complicate the code by keeping track of which memcgs were the ones
that actually enabled limits, and which ones got it from its parents.

It is a lot simpler just to do static_key_slow_inc() on every child
that is accounted.

[ v4: adapted this patch to the changes in kmem_accounted ]

Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
CC: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
CC: Christoph Lameter <cl@xxxxxxxxx>
CC: Pekka Enberg <penberg@xxxxxxxxxxxxxx>
CC: Michal Hocko <mhocko@xxxxxxx>
CC: Johannes Weiner <hannes@xxxxxxxxxxx>
CC: Suleiman Souhlal <suleiman@xxxxxxxxxx>

Looks reasonable to me
Acked-by: Michal Hocko <mhocko@xxxxxxx>

Just a little nit.


diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 634c7b5..724a08b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -344,11 +344,15 @@ struct mem_cgroup {
/* internal only representation about the status of kmem accounting. */
enum {
KMEM_ACCOUNTED_ACTIVE = 0, /* accounted by this cgroup itself */
+ KMEM_ACCOUNTED_ACTIVATED, /* static key enabled. */
KMEM_ACCOUNTED_DEAD, /* dead memcg, pending kmem charges */

-/* first bit */
+ * first two bits. We account when limit is on, but only after
+ * call sites are patched
+ */

The names are long but why not use KMEM_ACCOUNTED_ACTIVE*

Because the names are long! =)

please use "long" macros ;) it's not bad.


Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at