Messages in this thread | | | Date | Thu, 21 Jun 2018 13:51:44 +0800 | From | Feng Tang <> | Subject | Re: [PATCH v11 6/6] x86/tsc: use tsc early |
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Hi Pavel,
On Wed, Jun 20, 2018 at 05:27:00PM -0400, Pavel Tatashin wrote: > We want to get timestamps and high resultion clock available to us as early > as possible in boot. But, native_sched_clock() outputs time based either on > tsc after tsc_init() is called later in boot, or using jiffies when clock > interrupts are enabled, which is also happens later in boot. > > On the other hand, we know tsc frequency from as early as when > tsc_early_delay_calibrate() is called. So, we use the early tsc calibration > to output timestamps early. Later in boot when tsc_init() is called we > calibrate tsc again using more precise methods, and start using that. > > Since sched_clock() is in a hot path, we want to make sure that no > regressions are introduced to this function after machine is booted, this > is why we are using static branch that is enabled by default, but is > disabled once we have initialized a permanent clock source. > > Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com> > --- > arch/x86/kernel/tsc.c | 64 ++++++++++++++++++++++++++++++------------- > 1 file changed, 45 insertions(+), 19 deletions(-) > > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c > index 654a01cc0358..1dd69612c69c 100644 > --- a/arch/x86/kernel/tsc.c > +++ b/arch/x86/kernel/tsc.c > @@ -39,6 +39,9 @@ EXPORT_SYMBOL(tsc_khz); > static int __read_mostly tsc_unstable; > > static DEFINE_STATIC_KEY_FALSE(__use_tsc); > +static DEFINE_STATIC_KEY_TRUE(tsc_early_enabled);
Do we still need add a static_key? after Peter worked out the patch to enable ealy jump_label_init?
Thanks, Feng
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