lkml.org 
[lkml]   [2019]   [May]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics
On Tue, May 21, 2019 at 02:40:50PM -0700, kan.liang@linux.intel.com wrote:
> The 8bit metrics ratio values lose precision when the measurement period
> gets longer.
>
> To avoid this we always reset the metric value when reading, as we
> already accumulate the count in the perf count value.
>
> For a long period read, low precision is acceptable.
> For a short period read, the register will be reset often enough that it
> is not a problem.

> The PERF_METRICS may report wrong value if its delta was less than 1/255
> of SLOTS (Fixed counter 3).
>
> To avoid this, the PERF_METRICS and SLOTS registers have to be reset
> simultaneously. The slots value has to be cached as well.

That doesn't sound like it is NMI-safe.



> RDPMC
> =========
> The TopDown can be collected per thread/process. To use TopDown
> through RDPMC in applications on Icelake, the metrics and slots values
> have to be saved/restored during context switching.
>
> Add specific set_period() to specially handle the slots and metrics
> event. Because,
> - The initial value must be 0.
> - Only need to restore the value in context switch. For other cases,
> the counters have been cleared after read.

So the above claims to explain RDPMC, but doesn't mention that magic
value below at all. In fact, I don't see how the above relates to RDPMC
at all.

> @@ -2141,7 +2157,9 @@ static int x86_pmu_event_idx(struct perf_event *event)
> if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
> return 0;
>
> - if (x86_pmu.num_counters_fixed && idx >= INTEL_PMC_IDX_FIXED) {
> + if (is_metric_idx(idx))
> + idx = 1 << 29;

I can't find this in the SDM RDPMC description. What does it return?

> + else if (x86_pmu.num_counters_fixed && idx >= INTEL_PMC_IDX_FIXED) {
> idx -= INTEL_PMC_IDX_FIXED;
> idx |= 1 << 30;
> }

\
 
 \ /
  Last update: 2019-05-28 14:44    [W:0.183 / U:1.684 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site