lkml.org 
[lkml]   [2018]   [Jun]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v10 7/7] x86/tsc: use tsc early
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.

> > /*
> > * Scheduler clock - returns current time in nanosec units.
> > */
> > u64 native_sched_clock(void)
> > {
> > - if (static_branch_likely(&__use_tsc)) {
> > - u64 tsc_now = rdtsc();
> > + if (static_branch_likely(&__use_tsc))
> > + return cycles_2_ns(rdtsc());
> >
> > - /* return the value in ns */
> > - return cycles_2_ns(tsc_now);
> > + if (static_branch_unlikely(&tsc_early_enabled)) {
> > + if (tsc_early_sched_clock)
> > + return cycles_2_ns(rdtsc());
> > }

Thanks,

tglx

\
 
 \ /
  Last update: 2018-06-20 12:17    [W:0.132 / U:0.064 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site