Re: [PATCH v4 09/14] memcg: kmem accounting lifecycle management
From: Michal Hocko <hidden>
Date: 2012-10-11 13:11:48
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linux-mm, lkml
On Mon 08-10-12 14:06:15, Glauber Costa wrote:
Because kmem charges can outlive the cgroup, we need to make sure that we won't free the memcg structure while charges are still in flight. For reviewing simplicity, the charge functions will issue mem_cgroup_get() at every charge, and mem_cgroup_put() at every uncharge. This can get expensive, however, and we can do better. mem_cgroup_get() only really needs to be issued once: when the first limit is set. In the same spirit, we only need to issue mem_cgroup_put() when the last charge is gone. We'll need an extra bit in kmem_accounted for that: KMEM_ACCOUNTED_DEAD. it will be set when the cgroup dies, if there are charges in the group. If there aren't, we can proceed right away. Our uncharge function will have to test that bit every time the charges drop to 0. Because that is not the likely output of res_counter_uncharge, this should not impose a big hit on us: it is certainly much better than a reference count decrease at every operation. [ v3: merged all lifecycle related patches in one ] Signed-off-by: Glauber Costa <redacted> CC: Kamezawa Hiroyuki <redacted> CC: Christoph Lameter <redacted> CC: Pekka Enberg <penberg-bbCR+/B0CizivPeTLB3BmA@public.gmane.org> CC: Michal Hocko <redacted> CC: Johannes Weiner <redacted> CC: Suleiman Souhlal <redacted>
OK, I like the optimization. I have just one comment to the memcg_kmem_dead naming but other than that Acked-by: Michal Hocko <redacted> [...]
+static bool memcg_kmem_dead(struct mem_cgroup *memcg)
The name is tricky because it doesn't tell you that it clears the flag which made me scratch my head when reading comment in kmem_cgroup_destroy
+{
+ return test_and_clear_bit(KMEM_ACCOUNTED_DEAD, &memcg->kmem_accounted);
+}
#endif
/* Stuffs for move charges at task migration. */[...]
quoted hunk ↗ jump to hunk
@@ -4876,6 +4904,20 @@ static int memcg_init_kmem(struct mem_cgroup *memcg, struct cgroup_subsys *ss) static void kmem_cgroup_destroy(struct mem_cgroup *memcg) { mem_cgroup_sockets_destroy(memcg); + + memcg_kmem_mark_dead(memcg); + + if (res_counter_read_u64(&memcg->kmem, RES_USAGE) != 0) + return; + + /* + * Charges already down to 0, undo mem_cgroup_get() done in the charge + * path here, being careful not to race with memcg_uncharge_kmem: it is + * possible that the charges went down to 0 between mark_dead and the + * res_counter read, so in that case, we don't need the put + */ + if (memcg_kmem_dead(memcg)) + mem_cgroup_put(memcg);
-- Michal Hocko SUSE Labs