From mboxrd@z Thu Jan 1 00:00:00 1970 From: akpm@linux-foundation.org Subject: [to-be-updated] =?US-ASCII?Q?mm-memcg-avoid-stale-protection-values-when-cgroup-is-above-?= =?US-ASCII?Q?protection.patch?= removed from -mm tree Date: Tue, 05 May 2020 12:54:58 -0700 Message-ID: <20200505195458.kozMxY4l3%akpm@linux-foundation.org> Reply-To: linux-kernel@vger.kernel.org Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Return-path: Received: from mail.kernel.org ([198.145.29.99]:54870 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726350AbgEETy7 (ORCPT ); Tue, 5 May 2020 15:54:59 -0400 Sender: mm-commits-owner@vger.kernel.org List-Id: mm-commits@vger.kernel.org To: chris@chrisdown.name, guro@fb.com, hannes@cmpxchg.org, laoar.shao@gmail.com, mhocko@suse.com, mm-commits@vger.kernel.org The patch titled Subject: mm, memcg: avoid stale protection values when cgroup is above protection has been removed from the -mm tree. Its filename was mm-memcg-avoid-stale-protection-values-when-cgroup-is-above-protection.patch This patch was dropped because an updated version will be merged ------------------------------------------------------ From: Yafang Shao Subject: mm, memcg: avoid stale protection values when cgroup is above protection Patch series "mm: memcontrol: memory.{low,min} reclaim fix & cleanup". This series contains a fix for a edge case in my earlier protection calculation patches, and a patch to make the area overall a little more robust to hopefully help avoid this in future. This patch (of 2): A cgroup can have both memory protection and a memory limit to isolate it from its siblings in both directions - for example, to prevent it from being shrunk below 2G under high pressure from outside, but also from growing beyond 4G under low pressure. Commit 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim") implemented proportional scan pressure so that multiple siblings in excess of their protection settings don't get reclaimed equally but instead in accordance to their unprotected portion. During limit reclaim, this proportionality shouldn't apply of course: there is no competition, all pressure is from within the cgroup and should be applied as such. Reclaim should operate at full efficiency. However, mem_cgroup_protected() never expected anybody to look at the effective protection values when it indicated that the cgroup is above its protection. As a result, a query during limit reclaim may return stale protection values that were calculated by a previous reclaim cycle in which the cgroup did have siblings. When this happens, reclaim is unnecessarily hesitant and potentially slow to meet the desired limit. In theory this could lead to premature OOM kills, although it's not obvious this has occurred in practice. Workaround the problem by special casing reclaim roots in mem_cgroup_protection. These memcgs are never participating in the reclaim protection because the reclaim is internal. We have to ignore effective protection values for reclaim roots because mem_cgroup_protected might be called from racing reclaim contexts with different roots. Calculation is relying on root -> leaf tree traversal therefore top-down reclaim protection invariants should hold. The only exception is the reclaim root which should have effective protection set to 0 but that would be problematic for the following setup: Let's have global and A's reclaim in parallel: | A (low=2G, usage = 3G, max = 3G, children_low_usage = 1.5G) |\ | C (low = 1G, usage = 2.5G) B (low = 1G, usage = 0.5G) for A reclaim we have B.elow = B.low C.elow = C.low For the global reclaim A.elow = A.low B.elow = min(B.usage, B.low) because children_low_usage <= A.elow C.elow = min(C.usage, C.low) With the effective values resetting we have A reclaim A.elow = 0 B.elow = B.low C.elow = C.low and global reclaim could see the above and then B.elow = C.elow = 0 because children_low_usage > A.elow Which means that protected memcgs would get reclaimed. In future we would like to make mem_cgroup_protected more robust against racing reclaim contexts but that is likely more complex solution than this simple workaround. [hannes@cmpxchg.org - large part of the changelog] [mhocko@suse.com - workaround explanation] [hannes@cmpxchg.org: rework code comment] [hannes@cmpxchg.org: changelog] [chris@chrisdown.name: fix store tear] [chris@chrisdown.name: retitle] Link: http://lkml.kernel.org/r/cover.1588092152.git.chris@chrisdown.name Link: http://lkml.kernel.org/r/d454fca5d6b38b74d8dc35141e8519b02089a698.1588092152.git.chris@chrisdown.name Fixes: 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim") Signed-off-by: Yafang Shao Signed-off-by: Chris Down Acked-by: Johannes Weiner Acked-by: Michal Hocko Acked-by: Chris Down Acked-by: Roman Gushchin Signed-off-by: Andrew Morton --- mm/memcontrol.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) --- a/mm/memcontrol.c~mm-memcg-avoid-stale-protection-values-when-cgroup-is-above-protection +++ a/mm/memcontrol.c @@ -6392,8 +6392,19 @@ enum mem_cgroup_protection mem_cgroup_pr if (!root) root = root_mem_cgroup; - if (memcg == root) + if (memcg == root) { + /* + * The cgroup is the reclaim root in this reclaim + * cycle, and therefore not protected. But it may have + * stale effective protection values from previous + * cycles in which it was not the reclaim root - for + * example, global reclaim followed by limit reclaim. + * Reset these values for mem_cgroup_protection(). + */ + WRITE_ONCE(memcg->memory.emin, 0); + WRITE_ONCE(memcg->memory.elow, 0); return MEMCG_PROT_NONE; + } usage = page_counter_read(&memcg->memory); if (!usage) _ Patches currently in -mm which might be from laoar.shao@gmail.com are mm-memcg-fix-error-return-value-of-mem_cgroup_css_alloc.patch mm-memcg-fix-inconsistent-oom-event-behavior.patch mm-memcg-add-workingset_restore-in-memorystat.patch