linux-kernel - Re: [PATCH v12 09/11] x86/tsc: prepare for early sched

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Date:   Tue, 26 Jun 2018 17:42:14 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
cc:     Steven Sistare <steven.sistare@...cle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        John Stultz <john.stultz@...aro.org>, sboyd@...eaurora.org,
        x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        mingo@...hat.com, hpa@...or.com, douly.fnst@...fujitsu.com,
        peterz@...radead.org, prarit@...hat.com, feng.tang@...el.com,
        Petr Mladek <pmladek@...e.com>, gnomes@...rguk.ukuu.org.uk,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH v12 09/11] x86/tsc: prepare for early sched_clock

Pavel,

first of all, sorry for my last outburst. I just was in a lousy mood after
staring into too much half baken stuff and failed to make myself stay away
from the computer.

On Sun, 24 Jun 2018, Thomas Gleixner wrote:
> On Sat, 23 Jun 2018, Pavel Tatashin wrote:
> And this early init sequence also needs to pull over the tsc adjust
> magic. So tsc_early_delay_calibrate() which should btw. be renamed to
> tsc_early_init() should have:
> 
> {
> 	cpu_khz = x86_platform.calibrate_cpu();
>         tsc_khz = x86_platform.calibrate_tsc();
> 
>         tsc_khz = tsc_khz ? : cpu_khz;
>         if (!tsc_khz)
>                 return;
> 
>         /* Sanitize TSC ADJUST before cyc2ns gets initialized */
>         tsc_store_and_check_tsc_adjust(true);
> 
> 	calc_lpj(tsc_khz);
> 		
> 	tsc_sched_clock_init();
> }

Peter made me look deeper into this and there are a few issues, which I
missed, depending on when some of the resources become available. So we
probably cannot hook all of this into tsc_early_delay_calibrate().

I have an idea how to distangle it and we'll end up in a staged approach,
which looks like this:

    1) Earliest one (not sure how early yet)

       Attempt to use MSR/CPUID. If not running on a hypervisor this can
       try the quick PIT calibration, but nothing else.

    2) Post init_hypervisor_platform()

       An attempt to use the hypervisor data can be made.

    3) Post early_acpi_boot_init()

       This can do PIT/HPET based calibration

    4) Post x86_dtb_init()

       PIT/PMTIMER based calibration

Once tsc_khz is known, no further attempts of calibration are made. I'll
look into that later tonight.

Thanks,

	tglx