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

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Date:   Tue, 18 Jul 2023 13:42:54 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     David Hildenbrand <david@...hat.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 18/07/2023 12:24, David Hildenbrand wrote:
> 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.


Like this?

	if (!pid) {
		prctl(PR_SET_PDEATHSIG, SIGHUP);
		/* Parent may have died before prctl so check now. */
		if (getppid() == 1)
			kill(getpid(), SIGHUP);
		access_mem(ptr);
	}

>