[PATCH v4 0/2] memcg: Fix test_memcg_min/low test failures

[PATCH v4 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.
v4:
 - Add "#ifdef CONFIG_MEMCG" directives around shrink_node_memcgs() to
   avoid compilation problem with !CONFIG_MEMCG configs.

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 to be used mainly
by cgroup v1 code.

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                                      | 10 ++++++++++
 tools/testing/selftests/cgroup/test_memcontrol.c | 11 ++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

-- 
2.48.1

[PATCH v4 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() by skipping the !usage case. The
"#ifdef CONFIG_MEMCG" directive is added to avoid problem with the
non-CONFIG_MEMCG case.

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                                      | 10 ++++++++++
 tools/testing/selftests/cgroup/test_memcontrol.c |  7 ++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b620d74b0f66..65dee0ad6627 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -5926,6 +5926,7 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
 	return inactive_lru_pages > pages_for_compaction;
 }
 
+#ifdef CONFIG_MEMCG
 static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 {
 	struct mem_cgroup *target_memcg = sc->target_mem_cgroup;
@@ -5963,6 +5964,10 @@ 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.
@@ -6004,6 +6009,11 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 		}
 	} while ((memcg = mem_cgroup_iter(target_memcg, memcg, partial)));
 }
+#else
+static inline void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
+{
+}
+#endif /* CONFIG_MEMCG */
 
 static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 {
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

Re: [PATCH v4 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()

[Johannes Weiner]

On Sun, Apr 06, 2025 at 09:41:58PM -0400, Waiman Long wrote:
> 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() by skipping the !usage case. The
> "#ifdef CONFIG_MEMCG" directive is added to avoid problem with the
> non-CONFIG_MEMCG case.
> 
> 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                                      | 10 ++++++++++
>  tools/testing/selftests/cgroup/test_memcontrol.c |  7 ++++++-
>  2 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b620d74b0f66..65dee0ad6627 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -5926,6 +5926,7 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
>  	return inactive_lru_pages > pages_for_compaction;
>  }
>  
> +#ifdef CONFIG_MEMCG
>  static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>  {
>  	struct mem_cgroup *target_memcg = sc->target_mem_cgroup;
> @@ -5963,6 +5964,10 @@ 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;

Please use mem_cgroup_usage() like I had originally suggested.

The !CONFIG_MEMCG case can be done like its root cgroup branch.

>  		if (mem_cgroup_below_min(target_memcg, memcg)) {
>  			/*
>  			 * Hard protection.
> @@ -6004,6 +6009,11 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>  		}
>  	} while ((memcg = mem_cgroup_iter(target_memcg, memcg, partial)));
>  }
> +#else
> +static inline void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
> +{
> +}
> +#endif /* CONFIG_MEMCG */

You made the entire reclaim path a nop for !CONFIG_MEMCG.

Re: [PATCH v4 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()

[Waiman Long]

On 4/7/25 10:24 AM, Johannes Weiner wrote:
> On Sun, Apr 06, 2025 at 09:41:58PM -0400, Waiman Long wrote:
>> 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() by skipping the !usage case. The
>> "#ifdef CONFIG_MEMCG" directive is added to avoid problem with the
>> non-CONFIG_MEMCG case.
>>
>> 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                                      | 10 ++++++++++
>>   tools/testing/selftests/cgroup/test_memcontrol.c |  7 ++++++-
>>   2 files changed, 16 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index b620d74b0f66..65dee0ad6627 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -5926,6 +5926,7 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
>>   	return inactive_lru_pages > pages_for_compaction;
>>   }
>>   
>> +#ifdef CONFIG_MEMCG
>>   static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>>   {
>>   	struct mem_cgroup *target_memcg = sc->target_mem_cgroup;
>> @@ -5963,6 +5964,10 @@ 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;
> Please use mem_cgroup_usage() like I had originally suggested.
>
> The !CONFIG_MEMCG case can be done like its root cgroup branch.
Will do that.
>
>>   		if (mem_cgroup_below_min(target_memcg, memcg)) {
>>   			/*
>>   			 * Hard protection.
>> @@ -6004,6 +6009,11 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>>   		}
>>   	} while ((memcg = mem_cgroup_iter(target_memcg, memcg, partial)));
>>   }
>> +#else
>> +static inline void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>> +{
>> +}
>> +#endif /* CONFIG_MEMCG */
> You made the entire reclaim path a nop for !CONFIG_MEMCG.

Yes, that is probably not right. Will fix that.

Cheers,
Longman

Re: [PATCH v4 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()

[Michal Koutný]

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Hi Waiman.

On Sun, Apr 06, 2025 at 09:41:58PM -0400, Waiman Long <longman@redhat.com> wrote:
 ...
> 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

I'd suggest updating also the header of the test for clarity and then
exempt the Child 2 ('E') conditionally from comparisons, something like:

@@ -380,10 +380,10 @@ static bool reclaim_until(const char *memcg, long goal);
  *
  * Then it checks actual memory usages and expects that:
  * A/B    memory.current ~= 50M
- * A/B/C  memory.current ~= 29M
- * A/B/D  memory.current ~= 21M
- * A/B/E  memory.current ~= 0
- * A/B/F  memory.current  = 0
+ * A/B/C  memory.current ~= 29M, memory.events:low > 0
+ * A/B/D  memory.current ~= 21M, memory.events:low > 0
+ * A/B/E  memory.current ~= 0,   memory.events:low not specified (==0 w/out memory_recursiveprot)
+ * A/B/F  memory.current  = 0,   memory.events:low == 0
  * (for origin of the numbers, see model in memcg_protection.m.)
  *
  * After that it tries to allocate more than there is
@@ -527,6 +527,7 @@ static int test_memcg_protection(const char *root, bool min)

        for (i = 0; i < ARRAY_SIZE(children); i++) {
                int no_low_events_index = 1;
+               int ignore_low_events_index = has_recursiveprot ? 2 : -1;
                long low, oom;

                oom = cg_read_key_long(children[i], "memory.events", "oom ");
@@ -534,6 +535,8 @@ static int test_memcg_protection(const char *root, bool min)

                if (oom)
                        goto cleanup;
+               if (i == ignore_low_events_index)
+                       continue;
                if (i <= no_low_events_index && low <= 0)
                        goto cleanup;
                if (i > no_low_events_index && low)

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Re: [PATCH v4 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()

[Waiman Long]

On 4/7/25 11:25 AM, Michal Koutný wrote:
> Hi Waiman.
>
> On Sun, Apr 06, 2025 at 09:41:58PM -0400, Waiman Long <longman@redhat.com> wrote:
>   ...
>> 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
> I'd suggest updating also the header of the test for clarity and then
> exempt the Child 2 ('E') conditionally from comparisons, something like:
>
> @@ -380,10 +380,10 @@ static bool reclaim_until(const char *memcg, long goal);
>    *
>    * Then it checks actual memory usages and expects that:
>    * A/B    memory.current ~= 50M
> - * A/B/C  memory.current ~= 29M
> - * A/B/D  memory.current ~= 21M
> - * A/B/E  memory.current ~= 0
> - * A/B/F  memory.current  = 0
> + * A/B/C  memory.current ~= 29M, memory.events:low > 0
> + * A/B/D  memory.current ~= 21M, memory.events:low > 0
> + * A/B/E  memory.current ~= 0,   memory.events:low not specified (==0 w/out memory_recursiveprot)
> + * A/B/F  memory.current  = 0,   memory.events:low == 0
>    * (for origin of the numbers, see model in memcg_protection.m.)

Sorry for the late reply. I think it is a good idea to update the header 
as well. This function is actually used by both test_memcg_low and 
test_memcg.min. So I will use low/min instead.

Cheers,
Longman

>    *
>    * After that it tries to allocate more than there is
> @@ -527,6 +527,7 @@ static int test_memcg_protection(const char *root, bool min)
>
>          for (i = 0; i < ARRAY_SIZE(children); i++) {
>                  int no_low_events_index = 1;
> +               int ignore_low_events_index = has_recursiveprot ? 2 : -1;
>                  long low, oom;
>
>                  oom = cg_read_key_long(children[i], "memory.events", "oom ");
> @@ -534,6 +535,8 @@ static int test_memcg_protection(const char *root, bool min)
>
>                  if (oom)
>                          goto cleanup;
> +               if (i == ignore_low_events_index)
> +                       continue;
>                  if (i <= no_low_events_index && low <= 0)
>                          goto cleanup;
>                  if (i > no_low_events_index && low)

Re: [PATCH v4 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()

[Andrew Morton]

On Fri, 11 Apr 2025 17:08:33 -0400 Waiman Long <llong@redhat.com> wrote:

> > - * A/B/F  memory.current  = 0
> > + * A/B/C  memory.current ~= 29M, memory.events:low > 0
> > + * A/B/D  memory.current ~= 21M, memory.events:low > 0
> > + * A/B/E  memory.current ~= 0,   memory.events:low not specified (==0 w/out memory_recursiveprot)
> > + * A/B/F  memory.current  = 0,   memory.events:low == 0
> >    * (for origin of the numbers, see model in memcg_protection.m.)
> 
> Sorry for the late reply. I think it is a good idea to update the header 
> as well. This function is actually used by both test_memcg_low and 
> test_memcg.min. So I will use low/min instead.

It appears that quite a few updates are expected for this series,
so I'll drop v4 from mm.git.

[PATCH v4 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