Re: [BUG] perf tests: Test converting perf time to TSC

[Peter Zijlstra]

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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