Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 31 May 2018 17:52:10 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Petr Mladek <pmladek@suse.com>
Cc:     Feng Tang <feng.tang@intel.com>, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        linux-kernel@vger.kernel.org, alek.du@intel.com,
        pasha.tatashin@oracle.com
Subject: Re: [RFC 2/2] x86, tsc: Enable clock for ealry printk timestamp
Message-ID: <20180531155210.GL12180@hirez.programming.kicks-ass.net>
References: <1527672059-6225-1-git-send-email-feng.tang@intel.com>
 <1527672059-6225-2-git-send-email-feng.tang@intel.com>
 <20180531135542.4j7w7bxsw43ydx3j@pathway.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180531135542.4j7w7bxsw43ydx3j@pathway.suse.cz>
User-Agent: Mutt/1.9.5 (2018-04-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.