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Date:   Fri, 15 Mar 2019 10:29:22 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Zhenzhong Duan <zhenzhong.duan@oracle.com>
Cc:     linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Waiman Long <longman@redhat.com>,
        Srinivas Eeda <srinivas.eeda@oracle.com>, x86@kernel.org
Subject: Re: [PATCH 2/2] Revert "x86/hpet: Reduce HPET counter read
 contention"
Message-ID: <20190315092922.GB6521@hirez.programming.kicks-ass.net>
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 <20190315092529.GU5996@hirez.programming.kicks-ass.net>
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On Fri, Mar 15, 2019 at 10:25:29AM +0100, Peter Zijlstra wrote:
> On Thu, Mar 14, 2019 at 04:42:12PM +0800, Zhenzhong Duan wrote:
> > This reverts commit f99fd22e4d4bc84880a8a3117311bbf0e3a6a9dc.
> > 
> > It's unnecessory after commit "acpi_pm: Fix bootup softlockup due to PMTMR
> > counter read contention", the simple HPET access code could be restored.
> > 
> > On a general system with good TSC, TSC is the final default clocksource.
> > So the potential performce loss is only at bootup stage before TSC
> > replacing HPET, we didn't observe obvious delay of bootup.
> 
> The timeline here is:
> 
>  - Len took out SKX from native_calibrate_tsc
>    b51120309348 ("x86/tsc: Fix erroneous TSC rate on Skylake Xeon")
> 
>    This causes the TSC to run through the calibration code, which
>    completes _after_ SMP bringup.
> 
>  - This then caused HPET to be used during SMP bringup, which resulted
>    in Waiman doing the patch you now propose removing.
> 
>    Because large (multi-socket) SKX machines would barely boot.
> 
>    f99fd22e4d4b ("x86/hpet: Reduce HPET counter read contention")

Damn, my memory tricked me.. I just checked the dates on those patches
and I got it in reverse.

Anyway, the point still stands, when TSC is wrecked we still need HPET.
And you don't see a difference with the revert because of
clocksource_tsc_early, which didn't exist at the time.