On Thu, Oct 03, 2013 at 01:13:45PM +0300, Adrian Hunter wrote:
> > Anyway; looking at this, why does time_zero have these different checks
> > from the other time bits?
> >
> > @@ -1897,6 +1898,11 @@ void arch_perf_update_userpage(struct
> > perf_event_mmap_page *userpg, u64 now)
> > userpg->time_mult = this_cpu_read(cyc2ns);
> > userpg->time_shift = CYC2NS_SCALE_FACTOR;
> > userpg->time_offset = this_cpu_read(cyc2ns_offset) - now;
> > +
> > + if (sched_clock_stable && !check_tsc_disabled()) {
> > + userpg->cap_usr_time_zero = 1;
> > + userpg->time_zero = this_cpu_read(cyc2ns_offset);
> > + }
> > }
> >
> > That doesn't make any kind of sense.. why is cyc2ns_offset differently
> > tested from cyc2ns itself?
>
> I am afraid I don't understand the scaling calculations
> so I don't know if they make any sense.
>
> cap_usr_time_zero (now cap_user_time_zero) means you can convert
> perf time to / from TSC. That only works if TSC is not disabled
> and sched_clock is stable (and you have constant, non-stop TSC)
>
> As far as I can tell, assuming the hardware is not broken,
> sched_clock will be stable unless something (BIOS) or someone
> (meddling user) has changed TSC manually.
Well all the scaling mess only applies to TSC.. furthermore note how
time_offset even uses cyc2ns_offset.
So I don't see any point in cap_user_time and cap_user_time_zero having
different conditions.
Also, I don't think that a cpu without cap_tsc will ever run perf code
so I don't see the point of testing tsc_disabled; if someone is mad
enough to boot with that he can bloody well keep the pieces.
Arguably the CONSTANT && NONSTOP test was to test the same thing as
sched_clock_stable, but I suppose we can use that.
Which reminds me; I should go and fix the transition for
sched_clock_stable: 1->0.
---
diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 897783b3302a..9db2b361a63d 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -1888,21 +1888,16 @@ void arch_perf_update_userpage(struct
perf_event_mmap_page *userpg, u64 now)
userpg->cap_user_rdpmc = x86_pmu.attr_rdpmc;
userpg->pmc_width = x86_pmu.cntval_bits;
- if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
- return;
-
- if (!boot_cpu_has(X86_FEATURE_NONSTOP_TSC))
- return;
+ if (!sched_clock_stable)
+ return
userpg->cap_user_time = 1;
userpg->time_mult = this_cpu_read(cyc2ns);
userpg->time_shift = CYC2NS_SCALE_FACTOR;
userpg->time_offset = this_cpu_read(cyc2ns_offset) - now;
- if (sched_clock_stable && !check_tsc_disabled()) {
- userpg->cap_user_time_zero = 1;
- userpg->time_zero = this_cpu_read(cyc2ns_offset);
- }
+ userpg->cap_user_time_zero = 1;
+ userpg->time_zero = this_cpu_read(cyc2ns_offset);
}
/*
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