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

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Message-ID: <YFCiycIaViYCy3GH@hirez.programming.kicks-ass.net>
Date:   Tue, 16 Mar 2021 13:21:29 +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 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?