linux-kernel - Re: [RFC PATCH 2/2] lib: debugobjects: touch watchdog to avoid softlockup when !CONFIG

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Message-ID: <c3424f9e-5926-7eef-2063-9eb92980acb2@redhat.com>
Date:   Mon, 27 Nov 2017 14:36:52 -0500
From:   Waiman Long <longman@...hat.com>
To:     Yang Shi <yang.s@...baba-inc.com>, tglx@...utronix.de
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] lib: debugobjects: touch watchdog to avoid
 softlockup when !CONFIG_PREEMPT

On 11/27/2017 01:52 PM, Yang Shi wrote:
>
>
> On 11/27/17 10:18 AM, Waiman Long wrote:
>> On 11/27/2017 12:54 PM, Yang Shi wrote:
>>> Hi Waiman,
>>>
>>> The second patch of this series.
>>>
>>> Thanks,
>>> Yang
>>>
>>>
>>> On 11/17/17 11:43 AM, Yang Shi wrote:
>>>> There are nested loops on debug objects free path, sometimes it may
>>>> take
>>>> over hundred thousands of loops, then cause soft lockup with
>>>> !CONFIG_PREEMPT
>>>> occasionally, like below:
>>>>
>>>>   ...
>>>>
>>>> The code path might be called in either atomic or non-atomic context,
>>>> so touching softlockup watchdog instead of calling cond_resched()
>>>> which
>>>> might fall asleep. However, it is unnecessary to touch the watchdog
>>>> every loop, so just touch the watchdog at every 10000 (best estimate)
>>>> loops.
>>>>
>>>> Signed-off-by: Yang Shi <yang.s@...baba-inc.com>
>>
>> I do have some concern about suppressing the soft lockup warning
>> entirely. If the system feels unresponsive for a certain period of time
>> (e.g. 22s), most users would like to know what is going on. It can be a
>> custom message with less scary warning. Alternatively, some opt-out
>
> I'm not sure if it is necessary for debug code since the
> unresponsiveness is introduced by debug config and is expected
> somehow, so the user is supposed to know what they are doing, and it
> sounds preferred to disregard the soft lockup message reported by
> object debug for the most time.
>

Yes, it may be normal to cause the soft lockup while running some kind
of stress tests. However, it isn't normal for a real user application to
cause the lockup on a debug kernel. That is why I am suggesting some
kind of opt-out mechanism so that one can turn off the soft lockup
warning for stress testing.

> We do have some other examples which suppress soft lockup completely
> in kernel, i.e. kdb debug, printing some verbose debug or error
> message, some slow driver code, etc.
>
>> mechanism can be added to explicitly disable soft lookup warning for
>> debugobjs is OK as long as it is not the default.
> If we really want to some opt-out, we should be able to add a proc
> knob to disable soft lockup as the patch does, but not default. If
> this is too overkilling, we may just add some comment in the Kconfig
> help text to tell users the side effect. 

You may add a new debugfs file under the debug_objects sub-directory for
suppressing the soft lockup message.

Cheers,
Longman