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

[Thomas Gleixner]

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@xxxxxxxxxx

Thanks,

	tglx