Re: [tip:perf/core] perf/x86/intel: Clean-up/reduce PEBS code

Peter Zijlstra
2013-09-16T07:24:44Z

On Mon, Sep 16, 2013 at 08:07:36AM +0200, Peter Zijlstra wrote:
> There already was an implicit division there, and
> sizeof(pebs_record_hsw) = 176, can it really optimize that constant
> division?
>
> I suppose we could go and introduce CONFIG_PERF_DEBUG and stuff sanity
> checks under that.. :/

Or we could write it like so:

---
--- a/arch/x86/kernel/cpu/perf_event_intel_ds.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c
@@ -954,16 +954,16 @@ static void intel_pmu_drain_pebs_nhm(str

ds->pebs_index = ds->pebs_buffer_base;

- n = (top - at) / x86_pmu.pebs_record_size;
- if (n <= 0)
+ if (unlikely(at > top))
return;

/*
* Should not happen, we program the threshold at 1 and do not
* set a reset value.
*/
- WARN_ONCE(n > x86_pmu.max_pebs_events,
- "Unexpected number of pebs records %d\n", n);
+ WARN_ONCE(top - at > x86_pmu.max_pebs_events * x86_pmu.pebs_record_size,
+ "Unexpected number of pebs records %d\n",
+ (top - at) / x86_pmu.pebs_record_size);

for (; at < top; at += x86_pmu.pebs_record_size) {
struct pebs_record_nhm *p = at;