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

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Message-ID: <20180531155210.GL12180@hirez.programming.kicks-ass.net>
Date:   Thu, 31 May 2018 17:52:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Feng Tang <feng.tang@...el.com>, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        linux-kernel@...r.kernel.org, alek.du@...el.com,
        pasha.tatashin@...cle.com
Subject: Re: [RFC 2/2] x86, tsc: Enable clock for ealry printk timestamp

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.