linux-kernel - Re: [PATCH v2 6/8] selftests/mm: Make migration test robust to failure

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Date:   Tue, 18 Jul 2023 13:24:52 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Ryan Roberts <ryan.roberts@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Mark Brown <broonie@...nel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Florent Revest <revest@...omium.org>,
        Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 6/8] selftests/mm: Make migration test robust to
 failure

On 18.07.23 13:23, David Hildenbrand wrote:
> On 18.07.23 12:49, Ryan Roberts wrote:
>> On 17/07/2023 18:40, David Hildenbrand wrote:
>>> On 17.07.23 12:31, Ryan Roberts wrote:
>>>> The `migration` test currently has a number of robustness problems that
>>>> cause it to hang and leak resources.
>>>>
>>>> Timeout: There are 3 tests, which each previously ran for 60 seconds.
>>>> However, the timeout in mm/settings for a single test binary was set to
>>>> 45 seconds. So when run using run_kselftest.sh, the top level timeout
>>>> would trigger before the test binary was finished. Solve this by meeting
>>>> in the middle; each of the 3 tests now runs for 20 seconds (for a total
>>>> of 60), and the top level timeout is set to 90 seconds.
>>>>
>>>> Leaking child processes: the `shared_anon` test fork()s some children
>>>> but then an ASSERT() fires before the test kills those children. The
>>>> assert causes immediate exit of the parent and leaking of the children.
>>>> Furthermore, if run using the run_kselftest.sh wrapper, the wrapper
>>>> would get stuck waiting for those children to exit, which never happens.
>>>> Solve this by deferring any asserts until after the children are killed.
>>>> The same pattern is used for the threaded tests for uniformity.
>>>>
>>>> With these changes, the test binary now runs to completion on arm64,
>>>> with 2 tests passing and the `shared_anon` test failing.
>>>>
>>>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>>>> ---
>>>>     tools/testing/selftests/mm/migration.c | 14 ++++++++++----
>>>>     tools/testing/selftests/mm/settings    |  2 +-
>>>>     2 files changed, 11 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/tools/testing/selftests/mm/migration.c
>>>> b/tools/testing/selftests/mm/migration.c
>>>> index 379581567f27..189d7d9070e8 100644
>>>> --- a/tools/testing/selftests/mm/migration.c
>>>> +++ b/tools/testing/selftests/mm/migration.c
>>>> @@ -15,7 +15,7 @@
>>>>     #include <time.h>
>>>>       #define TWOMEG (2<<20)
>>>> -#define RUNTIME (60)
>>>> +#define RUNTIME (20)
>>>>       #define ALIGN(x, a) (((x) + (a - 1)) & (~((a) - 1)))
>>>>     @@ -118,6 +118,7 @@ TEST_F_TIMEOUT(migration, private_anon, 2*RUNTIME)
>>>>     {
>>>>         uint64_t *ptr;
>>>>         int i;
>>>> +    int ret;
>>>>           if (self->nthreads < 2 || self->n1 < 0 || self->n2 < 0)
>>>>             SKIP(return, "Not enough threads or NUMA nodes available");
>>>> @@ -131,9 +132,10 @@ TEST_F_TIMEOUT(migration, private_anon, 2*RUNTIME)
>>>>             if (pthread_create(&self->threads[i], NULL, access_mem, ptr))
>>>>                 perror("Couldn't create thread");
>>>>     -    ASSERT_EQ(migrate(ptr, self->n1, self->n2), 0);
>>>> +    ret = migrate(ptr, self->n1, self->n2);
>>>>         for (i = 0; i < self->nthreads - 1; i++)
>>>>             ASSERT_EQ(pthread_cancel(self->threads[i]), 0);
>>>> +    ASSERT_EQ(ret, 0);
>>>
>>> Why is that required? This does not involve fork.
>>
>> It's not required. I was just trying to keep everything aligned to the same pattern.
>>
>>>
>>>>     }
>>>>       /*
>>>> @@ -144,6 +146,7 @@ TEST_F_TIMEOUT(migration, shared_anon, 2*RUNTIME)
>>>>         pid_t pid;
>>>>         uint64_t *ptr;
>>>>         int i;
>>>> +    int ret;
>>>>           if (self->nthreads < 2 || self->n1 < 0 || self->n2 < 0)
>>>>             SKIP(return, "Not enough threads or NUMA nodes available");
>>>> @@ -161,9 +164,10 @@ TEST_F_TIMEOUT(migration, shared_anon, 2*RUNTIME)
>>>>                 self->pids[i] = pid;
>>>>         }
>>>>     -    ASSERT_EQ(migrate(ptr, self->n1, self->n2), 0);
>>>> +    ret = migrate(ptr, self->n1, self->n2);
>>>>         for (i = 0; i < self->nthreads - 1; i++)
>>>>             ASSERT_EQ(kill(self->pids[i], SIGTERM), 0);
>>>> +    ASSERT_EQ(ret, 0);
>>>
>>>
>>> Might be cleaner to also:
>>
>> Or instead of? I agree this is neater, so will undo the moving of the ASSERT()
>> and rely on this prctl.
> 
> I was thinking about possible races when our parent process already
> quits before our child managed to set the prctl. prctl() won't do
> anything in that case, hmmmm.
> 
> But similarly, existing code might already trigger the migrate() + kill
> before the child processes even started to access_mem().
> 
> Racy :)
> 

Maybe what would work, is checking after the prctl() in the child if the 
parent is already gone.

-- 
Cheers,

David / dhildenb