The test_memcontrol selftest consistently fails its test_memcg_low
sub-test due to the fact that two of its test child cgroups which
have a memmory.low of 0 or an effective memory.low of 0 still have low
events generated for them since mem_cgroup_below_low() use the ">="
operator when comparing to elow.
The two failed use cases are as follows:
1) memory.low is set to 0, but low events can still be triggered and
so the cgroup may have a non-zero low event count. I doubt users are
looking for that as they didn't set memory.low at all.
2) memory.low is set to a non-zero value but the cgroup has no task in
it so that it has an effective low value of 0. Again it may have a
non-zero low event count if memory reclaim happens. This is probably
not a result expected by the users and it is really doubtful that
users will check an empty cgroup with no task in it and expecting
some non-zero event counts.
In the first case, even though memory.low isn't set, it may still have
some low protection if memory.low is set in the parent. So low event may
still be recorded. The test_memcontrol.c test has to be modified to
account for that.
For the second case, it really doesn't make sense to have non-zero low
event if the cgroup has 0 usage. So we need to skip this corner case
in shrink_node_memcgs().
With this patch applied, the test_memcg_low sub-test finishes
successfully without failure in most cases. Though both test_memcg_low
and test_memcg_min sub-tests may still fail occasionally if the
memory.current values fall outside of the expected ranges.
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Waiman Long <longman@redhat.com>
---
mm/vmscan.c | 4 ++++
tools/testing/selftests/cgroup/test_memcontrol.c | 7 ++++++-
2 files changed, 10 insertions(+), 1 deletion(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b620d74b0f66..2a2957b9dc99 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -5963,6 +5963,10 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
mem_cgroup_calculate_protection(target_memcg, memcg);
+ /* Skip memcg with no usage */
+ if (!page_counter_read(&memcg->memory))
+ continue;
+
if (mem_cgroup_below_min(target_memcg, memcg)) {
/*
* Hard protection.
diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index 16f5d74ae762..bab826b6b7b0 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -525,8 +525,13 @@ static int test_memcg_protection(const char *root, bool min)
goto cleanup;
}
+ /*
+ * Child 2 has memory.low=0, but some low protection is still being
+ * distributed down from its parent with memory.low=50M. So the low
+ * event count will be non-zero.
+ */
for (i = 0; i < ARRAY_SIZE(children); i++) {
- int no_low_events_index = 1;
+ int no_low_events_index = 2;
long low, oom;
oom = cg_read_key_long(children[i], "memory.events", "oom ");
--
2.48.1Hi Waiman,
kernel test robot noticed the following build errors:
[auto build test ERROR on tj-cgroup/for-next]
[also build test ERROR on akpm-mm/mm-everything linus/master v6.14 next-20250404]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]
url: https://github.com/intel-lab-lkp/linux/commits/Waiman-Long/mm-vmscan-Skip-memcg-with-usage-in-shrink_node_memcgs/20250406-104208
base: https://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git for-next
patch link: https://lore.kernel.org/r/20250406024010.1177927-2-longman%40redhat.com
patch subject: [PATCH v3 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()
config: arm-randconfig-001-20250406 (https://download.01.org/0day-ci/archive/20250406/202504061254.DqfqHfM7-lkp@intel.com/config)
compiler: clang version 21.0.0git (https://github.com/llvm/llvm-project 92c93f5286b9ff33f27ff694d2dc33da1c07afdd)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250406/202504061254.DqfqHfM7-lkp@intel.com/reproduce)
If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202504061254.DqfqHfM7-lkp@intel.com/
All errors (new ones prefixed by >>):
>> mm/vmscan.c:5929:32: error: incomplete definition of type 'struct mem_cgroup'
5929 | if (!page_counter_read(&memcg->memory))
| ~~~~~^
include/linux/mm_types.h:33:8: note: forward declaration of 'struct mem_cgroup'
33 | struct mem_cgroup;
| ^
1 error generated.
vim +5929 mm/vmscan.c
5890
5891 static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
5892 {
5893 struct mem_cgroup *target_memcg = sc->target_mem_cgroup;
5894 struct mem_cgroup_reclaim_cookie reclaim = {
5895 .pgdat = pgdat,
5896 };
5897 struct mem_cgroup_reclaim_cookie *partial = &reclaim;
5898 struct mem_cgroup *memcg;
5899
5900 /*
5901 * In most cases, direct reclaimers can do partial walks
5902 * through the cgroup tree, using an iterator state that
5903 * persists across invocations. This strikes a balance between
5904 * fairness and allocation latency.
5905 *
5906 * For kswapd, reliable forward progress is more important
5907 * than a quick return to idle. Always do full walks.
5908 */
5909 if (current_is_kswapd() || sc->memcg_full_walk)
5910 partial = NULL;
5911
5912 memcg = mem_cgroup_iter(target_memcg, NULL, partial);
5913 do {
5914 struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);
5915 unsigned long reclaimed;
5916 unsigned long scanned;
5917
5918 /*
5919 * This loop can become CPU-bound when target memcgs
5920 * aren't eligible for reclaim - either because they
5921 * don't have any reclaimable pages, or because their
5922 * memory is explicitly protected. Avoid soft lockups.
5923 */
5924 cond_resched();
5925
5926 mem_cgroup_calculate_protection(target_memcg, memcg);
5927
5928 /* Skip memcg with no usage */
> 5929 if (!page_counter_read(&memcg->memory))
5930 continue;
5931
5932 if (mem_cgroup_below_min(target_memcg, memcg)) {
5933 /*
5934 * Hard protection.
5935 * If there is no reclaimable memory, OOM.
5936 */
5937 continue;
5938 } else if (mem_cgroup_below_low(target_memcg, memcg)) {
5939 /*
5940 * Soft protection.
5941 * Respect the protection only as long as
5942 * there is an unprotected supply
5943 * of reclaimable memory from other cgroups.
5944 */
5945 if (!sc->memcg_low_reclaim) {
5946 sc->memcg_low_skipped = 1;
5947 continue;
5948 }
5949 memcg_memory_event(memcg, MEMCG_LOW);
5950 }
5951
5952 reclaimed = sc->nr_reclaimed;
5953 scanned = sc->nr_scanned;
5954
5955 shrink_lruvec(lruvec, sc);
5956
5957 shrink_slab(sc->gfp_mask, pgdat->node_id, memcg,
5958 sc->priority);
5959
5960 /* Record the group's reclaim efficiency */
5961 if (!sc->proactive)
5962 vmpressure(sc->gfp_mask, memcg, false,
5963 sc->nr_scanned - scanned,
5964 sc->nr_reclaimed - reclaimed);
5965
5966 /* If partial walks are allowed, bail once goal is reached */
5967 if (partial && sc->nr_reclaimed >= sc->nr_to_reclaim) {
5968 mem_cgroup_iter_break(target_memcg, memcg);
5969 break;
5970 }
5971 } while ((memcg = mem_cgroup_iter(target_memcg, memcg, partial)));
5972 }
5973
--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki
Hi Waiman,
kernel test robot noticed the following build errors:
[auto build test ERROR on tj-cgroup/for-next]
[also build test ERROR on akpm-mm/mm-everything linus/master v6.14 next-20250404]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]
url: https://github.com/intel-lab-lkp/linux/commits/Waiman-Long/mm-vmscan-Skip-memcg-with-usage-in-shrink_node_memcgs/20250406-104208
base: https://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git for-next
patch link: https://lore.kernel.org/r/20250406024010.1177927-2-longman%40redhat.com
patch subject: [PATCH v3 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()
config: arc-randconfig-002-20250406 (https://download.01.org/0day-ci/archive/20250406/202504061257.GMkEJUOs-lkp@intel.com/config)
compiler: arc-linux-gcc (GCC) 11.5.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250406/202504061257.GMkEJUOs-lkp@intel.com/reproduce)
If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202504061257.GMkEJUOs-lkp@intel.com/
All errors (new ones prefixed by >>):
mm/vmscan.c: In function 'shrink_node_memcgs':
>> mm/vmscan.c:5929:46: error: invalid use of undefined type 'struct mem_cgroup'
5929 | if (!page_counter_read(&memcg->memory))
| ^~
vim +5929 mm/vmscan.c
5890
5891 static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
5892 {
5893 struct mem_cgroup *target_memcg = sc->target_mem_cgroup;
5894 struct mem_cgroup_reclaim_cookie reclaim = {
5895 .pgdat = pgdat,
5896 };
5897 struct mem_cgroup_reclaim_cookie *partial = &reclaim;
5898 struct mem_cgroup *memcg;
5899
5900 /*
5901 * In most cases, direct reclaimers can do partial walks
5902 * through the cgroup tree, using an iterator state that
5903 * persists across invocations. This strikes a balance between
5904 * fairness and allocation latency.
5905 *
5906 * For kswapd, reliable forward progress is more important
5907 * than a quick return to idle. Always do full walks.
5908 */
5909 if (current_is_kswapd() || sc->memcg_full_walk)
5910 partial = NULL;
5911
5912 memcg = mem_cgroup_iter(target_memcg, NULL, partial);
5913 do {
5914 struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);
5915 unsigned long reclaimed;
5916 unsigned long scanned;
5917
5918 /*
5919 * This loop can become CPU-bound when target memcgs
5920 * aren't eligible for reclaim - either because they
5921 * don't have any reclaimable pages, or because their
5922 * memory is explicitly protected. Avoid soft lockups.
5923 */
5924 cond_resched();
5925
5926 mem_cgroup_calculate_protection(target_memcg, memcg);
5927
5928 /* Skip memcg with no usage */
> 5929 if (!page_counter_read(&memcg->memory))
5930 continue;
5931
5932 if (mem_cgroup_below_min(target_memcg, memcg)) {
5933 /*
5934 * Hard protection.
5935 * If there is no reclaimable memory, OOM.
5936 */
5937 continue;
5938 } else if (mem_cgroup_below_low(target_memcg, memcg)) {
5939 /*
5940 * Soft protection.
5941 * Respect the protection only as long as
5942 * there is an unprotected supply
5943 * of reclaimable memory from other cgroups.
5944 */
5945 if (!sc->memcg_low_reclaim) {
5946 sc->memcg_low_skipped = 1;
5947 continue;
5948 }
5949 memcg_memory_event(memcg, MEMCG_LOW);
5950 }
5951
5952 reclaimed = sc->nr_reclaimed;
5953 scanned = sc->nr_scanned;
5954
5955 shrink_lruvec(lruvec, sc);
5956
5957 shrink_slab(sc->gfp_mask, pgdat->node_id, memcg,
5958 sc->priority);
5959
5960 /* Record the group's reclaim efficiency */
5961 if (!sc->proactive)
5962 vmpressure(sc->gfp_mask, memcg, false,
5963 sc->nr_scanned - scanned,
5964 sc->nr_reclaimed - reclaimed);
5965
5966 /* If partial walks are allowed, bail once goal is reached */
5967 if (partial && sc->nr_reclaimed >= sc->nr_to_reclaim) {
5968 mem_cgroup_iter_break(target_memcg, memcg);
5969 break;
5970 }
5971 } while ((memcg = mem_cgroup_iter(target_memcg, memcg, partial)));
5972 }
5973
--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki
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