linux-kernel - Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case

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Message-Id: <863331ED-B04A-4B94-91A2-D34002C9CCDC@amacapital.net>
Date:   Tue, 18 Sep 2018 07:01:12 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     John Stultz <john.stultz@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Matt Rickard <matt@...trans.com.au>,
        Stephen Boyd <sboyd@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        devel@...uxdriverproject.org,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Juergen Gross <jgross@...e.com>
Subject: Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case


> On Sep 18, 2018, at 12:52 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
>> On Mon, 17 Sep 2018, John Stultz wrote:
>>> On Mon, Sep 17, 2018 at 12:25 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>> Also, I'm not entirely convinced that this "last" thing is needed at
>>> all.  John, what's the scenario under which we need it?
>> 
>> So my memory is probably a bit foggy, but I recall that as we
>> accelerated gettimeofday, we found that even on systems that claimed
>> to have synced TSCs, they were actually just slightly out of sync.
>> Enough that right after cycles_last had been updated, a read on
>> another cpu could come in just behind cycles_last, resulting in a
>> negative interval causing lots of havoc.
>> 
>> So the sanity check is needed to avoid that case.
> 
> Your memory serves you right. That's indeed observable on CPUs which
> lack TSC_ADJUST.
> 
> @Andy: Welcome to the wonderful world of TSC.
> 

Do we do better if we use signed arithmetic for the whole calculation? Then a small backwards movement would result in a small backwards result.  Or we could offset everything so that we’d have to go back several hundred ms before we cross zero.