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

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Message-ID: <YFDCOYstnDWPSWRU@hirez.programming.kicks-ass.net>
Date:   Tue, 16 Mar 2021 15:35:37 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.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 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.