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

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Message-ID: <alpine.DEB.2.21.1806201229530.10546@nanos.tec.linutronix.de>
Date:   Wed, 20 Jun 2018 12:30:58 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Feng Tang <feng.tang@...el.com>
cc:     Pavel Tatashin <pasha.tatashin@...cle.com>,
        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, pmladek@...e.com,
        gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v10 7/7] x86/tsc: use tsc early

On Wed, 20 Jun 2018, Feng Tang wrote:

> On Wed, Jun 20, 2018 at 12:16:36PM +0200, Thomas Gleixner wrote:
> > On Wed, 20 Jun 2018, Feng Tang wrote:
> > > On Wed, Jun 20, 2018 at 01:52:10AM +0200, Thomas Gleixner wrote:
> > > >  
> > > >  static DEFINE_STATIC_KEY_FALSE(__use_tsc);
> > > > +static DEFINE_STATIC_KEY_TRUE(tsc_early_enabled);
> > > 
> > > One potential problem may be the several static_keys used here,
> > > the "__use_tsc", the "__sched_clock_stable", it may not be used
> > > very early in boot phase. As the the static_branch_enable() will
> > > use pageing related code while the paging is not setup ready yet.
> > 
> > I know how static keys work and thats the reason for having the extra
> > conditional. The key is disabled at a point where paging is available.
> 
> Maybe I'm wrong, for printk timestamp, it uses the local_clock() thus
>     sched_clock_cpu()
> 	sched_clock_stable
> 	    check the static_key "__sched_clock_stable" which is
> 	    defined FALSE, and need a static_branch_enable() to
> 	    take effect

And that has nothing to do with this particular patch. Please read the rest
of the series, especially

   https://lkml.kernel.org/r/20180615174204.30581-5-pasha.tatashin@oracle.com

Thanks,

	tglx