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

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Date:	Thu, 3 Oct 2013 10:56:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Jiri Olsa <jolsa@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org, David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [BUG] perf tests: Test converting perf time to TSC

On Thu, Oct 03, 2013 at 11:42:46AM +0300, Adrian Hunter wrote:
> On 03/10/13 11:17, Jiri Olsa wrote:
> > On Wed, Oct 02, 2013 at 04:46:59PM +0300, Adrian Hunter wrote:
> >> On 02/10/13 16:23, Jiri Olsa wrote:
> >>> hi,
> >>> got a segfault in the tsc test on latest acme's tree.
> >>>
> >>> I'm dealing with some other issues right now, so just reporting ;-)
> >>
> >> The capability bits have changed positions.  You need to have:
> >>
> >> commit fa7315871046b9a4c48627905691dbde57e51033
> >> Author: Peter Zijlstra <peterz@...radead.org>
> >> Date:   Thu Sep 19 10:16:42 2013 +0200
> >>
> >>     perf: Fix capabilities bitfield compatibility in 'struct
> >> perf_event_mmap_page'
> > 
> > ok, I'll try that.. but anyway, the test should
> > not crash in account of missing kernel change 
> 
> No the ABI is broken in that case - better to crash.

No; neither case should crash.

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?
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