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 01:02:24 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Niklas Cassel <niklas.cassel@...s.com>, wim@...ana.be,
        edumazet@...gle.com, linux-watchdog@...r.kernel.org,
        linux-kernel@...r.kernel.org, niklass@...s.com,
        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 12:36 AM, Peter Zijlstra wrote:
> On Sun, Feb 19, 2017 at 08:46:28AM -0800, 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.
>>
>
> It does no such thing; the hrtimer interface is always available.
>

That is good to hear. Thanks for the correction.

Guenter