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

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Date:   Tue, 16 Mar 2021 14:37:03 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "Zhou Ti (x2019cwm)" <x2019cwm@...x.ca>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Yunfeng Ye <yeyunfeng@...wei.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length()
 from returning negative value

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.