linux-kernel - Re: [PATCH] watchdog: softdog: fire watchdog even if softirqs do not get to run

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Date:   Mon, 20 Feb 2017 07:31:36 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Niklas Cassel <niklas.cassel@...s.com>, wim@...ana.be,
        edumazet@...gle.com, peterz@...radead.org
Cc:     linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org,
        Wolfram Sang <wsa+renesas@...g-engineering.com>
Subject: Re: [PATCH] watchdog: softdog: fire watchdog even if softirqs do not
 get to run

On 02/20/2017 02:03 AM, Niklas Cassel wrote:
> On 02/19/2017 05:46 PM, Guenter Roeck wrote:
>> Cc: Wolfram for input.
>>
>> On 02/17/2017 10:25 AM, Niklas Cassel wrote:
>>> From: Niklas Cassel <niklas.cassel@...s.com>
>>>
>>> Checking for timer expiration is done from the softirq TIMER_SOFTIRQ.
>>>
>>> Since commit 4cd13c21b207 ("softirq: Let ksoftirqd do its job"),
>>> pending softirqs are no longer always handled immediately, instead,
>>> if there are pending softirqs, and ksoftirqd is in state TASK_RUNNING,
>>> the handling of the softirqs are deferred, and are instead supposed
>>> to be handled by ksoftirqd, when ksoftirqd gets scheduled.
>>>
>>> If a user space process with a real-time policy starts to misbehave
>>> by never relinquishing the CPU while ksoftirqd is in state TASK_RUNNING,
>>> what will happen is that all softirqs will get deferred, while ksoftirqd,
>>> which is supposed to handle the deferred softirqs, will never get to run.
>>>
>>> To make sure that the watchdog is able to fire even when we do not get
>>> to run softirqs, replace the timers with hrtimers.
>>>
>>
>> This makes the driver dependent on HIGH_RES_TIMERS, which is not available
>> on all architectures. Before adding that restriction, I would like to see
>> some discussion if this is the only feasible solution.
>>
>> Is this driver the only one with this problem, or is anything using
>> timers affected ?
>
> Anything using timers is affected.
> The timers will still get incremented, but the code checking for timer
> expiration is run from a softirq, which in this case never gets to run,
> so the timers will never expire.
>
> Before 4cd13c21b207 ("softirq: Let ksoftirqd do its job"), softirqs
> were never deferred, so they always got to run when exiting an irq.
>
> So previously with a user space process using all the CPU, like:
> chrt -r 99 sh -c "while :; do :; done"
> the softdog would still fire.
>
> If we ask the system to run something all the time,
> and the system does that, I don't think we can blame the system.
> It is however important that the watchdog can still detect and
> fire when this happens. Other drivers, not so much.
>
> I guess another solution would be to modify the if-statements in
> kernel/softirq.c to sometimes do the softirq directly, even if ksoftirqd
> is in state TASK_RUNNING, if we also meet some other condition.
> However, do we want to add that extra complexity?

No, in that respect using hrtimers is ok. I was more concerned about
other users of timers. But, as you say, I guess that is considered ok.

Guenter