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Date:   Tue, 16 Mar 2021 15:53:52 +0100
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        "Zhou Ti (x2019cwm)" <x2019cwm@stfx.ca>,
        Yunfeng Ye <yeyunfeng@huawei.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length()
 from returning negative value
Message-ID: <20210316145352.GE639918@lothringen>
References: <20210311123708.23501-1-frederic@kernel.org>
 <20210311123708.23501-2-frederic@kernel.org>
 <YFCiycIaViYCy3GH@hirez.programming.kicks-ass.net>
 <20210316133703.GC639918@lothringen>
 <YFDCOYstnDWPSWRU@hirez.programming.kicks-ass.net>
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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@stfx.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?