linux-kernel - Re: [RFC 2/2] x86, tsc: Enable clock for ealry printk timestamp

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Message-ID: <CAGM2rebzatqO20X4SmdH=VmVGmf-8Uo6FAB48p2zsuHFj-0X_w@mail.gmail.com>
Date:   Fri, 1 Jun 2018 14:05:33 -0400
From:   Pavel Tatashin <pasha.tatashin@...cle.com>
To:     peterz@...radead.org
Cc:     pmladek@...e.com, feng.tang@...el.com, mingo@...nel.org,
        tglx@...utronix.de, hpa@...or.com, gnomes@...rguk.ukuu.org.uk,
        LKML <linux-kernel@...r.kernel.org>, alek.du@...el.com
Subject: Re: [RFC 2/2] x86, tsc: Enable clock for ealry printk timestamp

Hi Peter,

How about taking patches 1-4 and 7 ? I can work on patches 5 & 6
separetly to provide a better integration for x86, this can be decided
separetly but the other patches will enable early clock functionality
on arches where sched_clock() is available early.

Pavel
On Thu, May 31, 2018 at 11:52 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, May 31, 2018 at 03:55:42PM +0200, Petr Mladek wrote:
> > I wonder if we could get some cleaner integration into the timer and
> > printk code.
>
> Yes, these patches are particularly horrific..
>
> There were some earlier patches by Pavel Tatashin, which attempted do
> get things running earlier.
>
>   http://lkml.kernel.org/r/20180209211143.16215-1-pasha.tatashin@oracle.com
>
> I'm not entirely happy with that, but I never did get around to
> reviewing that last version :-( In particuarly, now that you made me
> look, I dislike his patch 6 almost as much as these patches.
>
> The idea was to get regular sched_clock() running earlier, not to botch
> some early_sched_clock() into it.
>
> Basically run calibrate_tsc() earlier (like _waaay_ earlier, it doesn't
> rely on anything other than CPUID) and if you have a recent part (with
> exception of SKX) you'll get a usable tsc rate (and TSC_RELIABLE) and
> things will work.
>
> If you have a dodgy part (sorry SKX), you'll just have to live with
> sched_clock starting late(r).
>
> Do not cobble things on the side, try and get the normal things running
> earlier.