linux-kernel - Re: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep

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Date:   Tue, 16 Mar 2021 16:26:02 +0100
From:   "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
To:     Frederic Weisbecker <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "Zhou Ti (x2019cwm)" <x2019cwm@...x.ca>,
        Yunfeng Ye <yeyunfeng@...wei.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        "rafael@...nel.org" <rafael@...nel.org>
Subject: Re: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length()
 from returning negative value

On 3/16/2021 3:53 PM, Frederic Weisbecker wrote:
> On Tue, Mar 16, 2021 at 03:35:37PM +0100, Peter Zijlstra wrote:
>> On Tue, Mar 16, 2021 at 02:37:03PM +0100, Frederic Weisbecker wrote:
>>> On Tue, Mar 16, 2021 at 01:21:29PM +0100, Peter Zijlstra wrote:
>>>> On Thu, Mar 11, 2021 at 01:36:59PM +0100, Frederic Weisbecker wrote:
>>>>> From: "Zhou Ti (x2019cwm)" <x2019cwm@...x.ca>
>>>>>
>>>>> If the hardware clock happens to fire its interrupts late, two possible
>>>>> issues can happen while calling tick_nohz_get_sleep_length(). Either:
>>>>>
>>>>> 1) The next clockevent device event is due past the last idle entry time.
>>>>>
>>>>> or:
>>>>>
>>>>> 2) The last timekeeping update happened before the last idle entry time
>>>>>     and the next timer callback expires before the last idle entry time.
>>>>>
>>>>> Make sure that both cases are handled to avoid returning a negative
>>>>> duration to the cpuidle governors.
>>>> Why? ... and wouldn't it be cheaper the fix the caller to
>>>> check negative once, instead of adding two branches here?
>>> There are already two callers and potentially two return values to check
>>> for each because the function returns two values.
>>>
>>> I'd rather make the API more robust instead of fixing each callers and worrying
>>> about future ones.
>> But what's the actual problem? The Changelog doesn't say why returning a
>> negative value is a problem, and in fact the return value is explicitly
>> signed.
>>
>> Anyway, I don't terribly mind the patch, I was just confused by the lack
>> of actual justification.
> And you're right, the motivation is pure FUD: I don't know exactly
> how the cpuidle governors may react to such negative values and so this
> is just to prevent from potential accident.
>
> Rafael, does that look harmless to you?

No, this is a problem now.  Both governors using this assign the return 
value of it to a u64 var and so negative values confuse them.

That said I think it's better to deal with the issue in the callers.

I can send a patch for that if needed.