linux-kernel - Re: [PATCH 2/2] x86/idle: use dynamic halt poll

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Message-ID: <alpine.DEB.2.20.1707041648540.9000@nanos>
Date:   Tue, 4 Jul 2017 16:50:54 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Radim Krčmář <rkrcmar@...hat.com>
cc:     Yang Zhang <yang.zhang.wz@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Jonathan Corbet <corbet@....net>, tony.luck@...el.com,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>, mchehab@...nel.org,
        Andrew Morton <akpm@...ux-foundation.org>, krzk@...nel.org,
        jpoimboe@...hat.com, Andy Lutomirski <luto@...nel.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
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        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
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        kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/2] x86/idle: use dynamic halt poll

On Tue, 4 Jul 2017, Radim Krčmář wrote:
> I think there is a nicer solution to avoid the expensive timer rewrite:
> Linux uses one-shot APIC timers and getting the timer interrupt is about
> as expensive as programming the timer, so the guest can keep the timer
> armed, but not re-arm it after the expiration if the CPU is idle.

So much for the theory. The NOHZ logic has to reprogram the timer when the
next expiry is farther away than the next tick. Otherwise you wake up on
every idle entry once for nothing, which defeats the whole purpose of NOHZ
to not do that.

Thanks,

	tglx