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

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Message-ID: <1a6f92f5-2c12-1276-916b-fd2848899e5c@redhat.com>
Date:   Mon, 27 Nov 2017 13:18:36 -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 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
mechanism can be added to explicitly disable soft lookup warning for
debugobjs is OK as long as it is not the default.

Cheers,
Longman