[PATCH v3 0/2] memcg: Fix test_memcg_min/low test failures

[PATCH v3 0/2] memcg: Fix test_memcg_min/low test failures

[Waiman Long]

v3:
 - Take up Johannes' suggestion of just skip the !usage case and
   fix test_memcontrol selftest to fix the rests of the min/low
   failures.

The test_memcontrol selftest consistently fails its test_memcg_low
sub-test and sporadically fails its test_memcg_min sub-test. This
patchset fixes the test_memcg_min and test_memcg_low failures by
skipping the !usage case in shrink_node_memcgs() and adjust the
test_memcontrol selftest to fix other causes of the test failures.

Note that I decide not to use the suggested mem_cgroup_usage() call
as it is a real function call defined in mm/memcontrol.c which is not
available if CONFIG_MEMCG isn't defined.

Waiman Long (2):
  mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()
  selftests: memcg: Increase error tolerance of child memory.current
    check in test_memcg_protection()

 mm/vmscan.c                                      |  4 ++++
 tools/testing/selftests/cgroup/test_memcontrol.c | 11 ++++++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

-- 
2.48.1

[PATCH v3 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()

[Waiman Long]

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 ");
-- 
2.48.1

[PATCH v3 2/2] selftests: memcg: Increase error tolerance of child memory.current check in test_memcg_protection()

[Waiman Long]

The test_memcg_protection() function is used for the test_memcg_min and
test_memcg_low sub-tests. This function generates a set of parent/child
cgroups like:

  parent:  memory.min/low = 50M
  child 0: memory.min/low = 75M,  memory.current = 50M
  child 1: memory.min/low = 25M,  memory.current = 50M
  child 2: memory.min/low = 0,    memory.current = 50M

After applying memory pressure, the function expects the following
actual memory usages.

  parent:  memory.current ~= 50M
  child 0: memory.current ~= 29M
  child 1: memory.current ~= 21M
  child 2: memory.current ~= 0

In reality, the actual memory usages can differ quite a bit from the
expected values. It uses an error tolerance of 10% with the values_close()
helper.

Both the test_memcg_min and test_memcg_low sub-tests can fail
sporadically because the actual memory usage exceeds the 10% error
tolerance. Below are a sample of the usage data of the tests runs
that fail.

  Child   Actual usage    Expected usage    %err
  -----   ------------    --------------    ----
    1       16990208         22020096      -12.9%
    1       17252352         22020096      -12.1%
    0       37699584         30408704      +10.7%
    1       14368768         22020096      -21.0%
    1       16871424         22020096      -13.2%

The current 10% error tolerenace might be right at the time
test_memcontrol.c was first introduced in v4.18 kernel, but memory
reclaim have certainly evolved quite a bit since then which may result
in a bit more run-to-run variation than previously expected.

Increase the error tolerance to 15% for child 0 and 20% for child 1 to
minimize the chance of this type of failure. The tolerance is bigger
for child 1 because an upswing in child 0 corresponds to a smaller
%err than a similar downswing in child 1 due to the way %err is used
in values_close().

Before this patch, a 100 test runs of test_memcontrol produced the
following results:

     17 not ok 1 test_memcg_min
     22 not ok 2 test_memcg_low

After applying this patch, there were no test failure for test_memcg_min
and test_memcg_low in 100 test runs.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 tools/testing/selftests/cgroup/test_memcontrol.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index bab826b6b7b0..8f4f2479650e 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -495,10 +495,10 @@ static int test_memcg_protection(const char *root, bool min)
 	for (i = 0; i < ARRAY_SIZE(children); i++)
 		c[i] = cg_read_long(children[i], "memory.current");
 
-	if (!values_close(c[0], MB(29), 10))
+	if (!values_close(c[0], MB(29), 15))
 		goto cleanup;
 
-	if (!values_close(c[1], MB(21), 10))
+	if (!values_close(c[1], MB(21), 20))
 		goto cleanup;
 
 	if (c[3] != 0)
-- 
2.48.1

Re: [PATCH v3 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()

[kernel test robot]

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

Re: [PATCH v3 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()

[kernel test robot]

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: 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

Re: [PATCH v3 2/2] selftests: memcg: Increase error tolerance of child memory.current check in test_memcg_protection()

[Roman Gushchin]

On Sat, Apr 05, 2025 at 10:40:10PM -0400, Waiman Long wrote:
> The test_memcg_protection() function is used for the test_memcg_min and
> test_memcg_low sub-tests. This function generates a set of parent/child
> cgroups like:
> 
>   parent:  memory.min/low = 50M
>   child 0: memory.min/low = 75M,  memory.current = 50M
>   child 1: memory.min/low = 25M,  memory.current = 50M
>   child 2: memory.min/low = 0,    memory.current = 50M
> 
> After applying memory pressure, the function expects the following
> actual memory usages.
> 
>   parent:  memory.current ~= 50M
>   child 0: memory.current ~= 29M
>   child 1: memory.current ~= 21M
>   child 2: memory.current ~= 0
> 
> In reality, the actual memory usages can differ quite a bit from the
> expected values. It uses an error tolerance of 10% with the values_close()
> helper.
> 
> Both the test_memcg_min and test_memcg_low sub-tests can fail
> sporadically because the actual memory usage exceeds the 10% error
> tolerance. Below are a sample of the usage data of the tests runs
> that fail.
> 
>   Child   Actual usage    Expected usage    %err
>   -----   ------------    --------------    ----
>     1       16990208         22020096      -12.9%
>     1       17252352         22020096      -12.1%
>     0       37699584         30408704      +10.7%
>     1       14368768         22020096      -21.0%
>     1       16871424         22020096      -13.2%
> 
> The current 10% error tolerenace might be right at the time
> test_memcontrol.c was first introduced in v4.18 kernel, but memory
> reclaim have certainly evolved quite a bit since then which may result
> in a bit more run-to-run variation than previously expected.
> 
> Increase the error tolerance to 15% for child 0 and 20% for child 1 to
> minimize the chance of this type of failure. The tolerance is bigger
> for child 1 because an upswing in child 0 corresponds to a smaller
> %err than a similar downswing in child 1 due to the way %err is used
> in values_close().
> 
> Before this patch, a 100 test runs of test_memcontrol produced the
> following results:
> 
>      17 not ok 1 test_memcg_min
>      22 not ok 2 test_memcg_low
> 
> After applying this patch, there were no test failure for test_memcg_min
> and test_memcg_low in 100 test runs.

Ideally we want to calculate these values dynamically based on the machine
size (number of cpus and total memory size).

We can calculate the memcg error margin and scale memcg sizes if necessarily.
It's the only way to make it pass both on a 2-CPU's vm and 512-CPU's physical
server.

Not a blocker for this patch, just an idea for the future.

Thanks!

Re: [PATCH v3 2/2] selftests: memcg: Increase error tolerance of child memory.current check in test_memcg_protection()

[Waiman Long]

On 4/8/25 6:22 PM, Roman Gushchin wrote:
> On Sat, Apr 05, 2025 at 10:40:10PM -0400, Waiman Long wrote:
>> The test_memcg_protection() function is used for the test_memcg_min and
>> test_memcg_low sub-tests. This function generates a set of parent/child
>> cgroups like:
>>
>>    parent:  memory.min/low = 50M
>>    child 0: memory.min/low = 75M,  memory.current = 50M
>>    child 1: memory.min/low = 25M,  memory.current = 50M
>>    child 2: memory.min/low = 0,    memory.current = 50M
>>
>> After applying memory pressure, the function expects the following
>> actual memory usages.
>>
>>    parent:  memory.current ~= 50M
>>    child 0: memory.current ~= 29M
>>    child 1: memory.current ~= 21M
>>    child 2: memory.current ~= 0
>>
>> In reality, the actual memory usages can differ quite a bit from the
>> expected values. It uses an error tolerance of 10% with the values_close()
>> helper.
>>
>> Both the test_memcg_min and test_memcg_low sub-tests can fail
>> sporadically because the actual memory usage exceeds the 10% error
>> tolerance. Below are a sample of the usage data of the tests runs
>> that fail.
>>
>>    Child   Actual usage    Expected usage    %err
>>    -----   ------------    --------------    ----
>>      1       16990208         22020096      -12.9%
>>      1       17252352         22020096      -12.1%
>>      0       37699584         30408704      +10.7%
>>      1       14368768         22020096      -21.0%
>>      1       16871424         22020096      -13.2%
>>
>> The current 10% error tolerenace might be right at the time
>> test_memcontrol.c was first introduced in v4.18 kernel, but memory
>> reclaim have certainly evolved quite a bit since then which may result
>> in a bit more run-to-run variation than previously expected.
>>
>> Increase the error tolerance to 15% for child 0 and 20% for child 1 to
>> minimize the chance of this type of failure. The tolerance is bigger
>> for child 1 because an upswing in child 0 corresponds to a smaller
>> %err than a similar downswing in child 1 due to the way %err is used
>> in values_close().
>>
>> Before this patch, a 100 test runs of test_memcontrol produced the
>> following results:
>>
>>       17 not ok 1 test_memcg_min
>>       22 not ok 2 test_memcg_low
>>
>> After applying this patch, there were no test failure for test_memcg_min
>> and test_memcg_low in 100 test runs.
> Ideally we want to calculate these values dynamically based on the machine
> size (number of cpus and total memory size).
>
> We can calculate the memcg error margin and scale memcg sizes if necessarily.
> It's the only way to make it pass both on a 2-CPU's vm and 512-CPU's physical
> server.
>
> Not a blocker for this patch, just an idea for the future.

Thanks for the suggestion.

As I said in a previous mail, the way the test works is by waiting until 
the the memory.current of the parent is close to 50M, then it checks the 
memory.current's of its children to see how much usage each of them 
have. I am not sure if nr of CPUs or total memory size is really a 
factor here. We will probably need to run some experiments to find out. 
Anyway, it will be a future patch if they are really a factor here.

Cheers,
Longman

>
> Thanks!
>