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
