linux-kernel - Re: [PATCH v10 7/7] x86/tsc: use tsc early

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Date:   Wed, 20 Jun 2018 00:51:45 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, john.stultz@...aro.org,
        sboyd@...eaurora.org, x86@...nel.org, 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,
        pmladek@...e.com, gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v10 7/7] x86/tsc: use tsc early

On Fri, 15 Jun 2018, Pavel Tatashin wrote:
> This patch adds early clock feature to x86 platforms.

See Documentation about 'This patch' .... We already know that this is a
patch otherwise it would not be marked as such ...

> +/*
> + * Finish clock for early time stamps, and hand over to permanent clock by
> + * setting __sched_clock_offset appropriately for continued time keeping.
> + */
> +static void __init tsc_early_fini(void)
> +{
> +	unsigned long long t;
> +	unsigned long r;
> +
> +	t = -cyc2ns_early.cyc2ns_offset;
> +	r = do_div(t, NSEC_PER_SEC);
> +
> +	__sched_clock_offset = sched_clock_early() - sched_clock();

As this is called _AFTER_ the TSC initialization which also includes
sanitizing of TSC_ADJUST, this can result in a complete bogus
__sched_clock_offset because sched_clock_early() will return nonsense when
TSC_ADJUST has been modified.

> +	pr_info("early sched clock is finished, offset [%lld.%09lds]\n", t, r);

That only gives useful information when the machine boots cold and when the
machine has a sane BIOS. So it's almost guaranteed to print useless
information.

Thanks,

	tglx