Re: [PATCH] rcu-tasks: Delay rcu_tasks_verify_self_tests() to avoid missed callbacks

[Waiman Long <longman@redhat.com>]

On 6/13/22 13:56, Paul E. McKenney wrote:
> On Mon, Jun 13, 2022 at 12:01:24PM -0400, Waiman Long wrote:
>> On 6/10/22 16:58, Paul E. McKenney wrote:
>>> On Fri, Jun 10, 2022 at 02:42:12PM -0400, Waiman Long wrote:
>>>> Even though rcu_tasks selftest is initiated early in the boot process,
>>>> the verification done at late initcall time may not be late enough to
>>>> catch all the callbacks especially on systems with just a few cpus and
>>>> small memory.
>>>>
>>>> After 12 bootup's On a s390x system, 1 of them had failed rcu_tasks
>>>> verification test.
>>>>
>>>> [    8.183013] call_rcu_tasks() has been failed.
>>>> [    8.183041] WARNING: CPU: 0 PID: 1 at kernel/rcu/tasks.h:1696 rcu_tasks_verify_self_tests+0x64/0xd0
>>>> [    8.203246] Callback from call_rcu_tasks() invoked.
>>>>
>>>> In this particular case, the callback missed the check by about
>>>> 20ms. Similar rcu_tasks selftest failures are also seen in ppc64le
>>>> systems.
>>>>
>>>> [    0.313391] call_rcu_tasks() has been failed.
>>>> [    0.313407] WARNING: CPU: 0 PID: 1 at kernel/rcu/tasks.h:1696 rcu_tasks_verify_self_tests+0x5c/0xa0
>>>> [    0.335569] Callback from call_rcu_tasks() invoked.
>>>>
>>>> Avoid this missed callback by delaying the verification using
>>>> delayed_work. The delay is set to be about 0.1s which hopefully will
>>>> be long enough to catch all the callbacks on systems with few cpus and
>>>> small memory.
>>>>
>>>> Fixes: bfba7ed084f8 ("rcu-tasks: Add RCU-tasks self tests")
>>>> Signed-off-by: Waiman Long <longman@redhat.com>
>>> Good catch, thank you!
>>>
>>> A few days ago, I queued this:
>>>
>>> 2585014188d5 ("rcu-tasks: Be more patient for RCU Tasks boot-time testing")
>>>
>>> This is shown in full at the end of this email.  Does this fix this
>>> problem for you?
>> I think your patch should fix the false positive warning and it give plenty
>> of time for this to happen.
>>
>> I do have one question though. rcu_tasks_verify_selft_tests() is called from
>> do_initcalls(). Since it may not be the last late initcall, does that mean
>> other late initcalls queued after that may be delayed by a second or more?
> Indeed.  Which is why I would welcome the workqueues portion of your
> patch on top of the above patch in -rcu.  ;-)

Sure. I will work on such a follow-up patch.

Cheers,
Longman