Re: [PATCH] Update perf tools topdown documentation

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2024-12-11 1:20 p.m., Ian Rogers wrote:
> On Wed, Dec 11, 2024 at 8:07 AM Liang, Kan <kan.liang@linux.intel.com> wrote:
>>> The RDPMC can automatically reset the perf metrics.
>>> Maybe change it to "automatically resetting the perf metrics RDPMC"
>>>
>>
>> The new post can be found at
>> https://lore.kernel.org/lkml/20241211160318.235056-2-kan.liang@linux.intel.com/
>>
>> I also added the reviewed-by in the new patch.
>>
>> Please let me know if there are more comments.
> 
> Hi Kan,
> 
> Vince Weaver posted to the list a related question on topdown + rdpmc + hybrid:
> https://lore.kernel.org/linux-perf-users/b0b0ae91-785e-64f6-348a-ca6edd448b74@maine.edu/
> The topic of being rescheduled isn't part of the rdpmc documentation:
> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/include/uapi/linux/perf_event.h?h=perf-tools-next#n610

Maybe we should emphasis that the example in the documentation is only
for p-core.

diff --git a/tools/perf/Documentation/topdown.txt
b/tools/perf/Documentation/topdown.txt
index 44644e917ffd..a229d0043399 100644
--- a/tools/perf/Documentation/topdown.txt
+++ b/tools/perf/Documentation/topdown.txt
@@ -21,14 +21,14 @@ per architecture.
      5.704818971                 14.0                 27.5
    51.3                    7.3
 ...

-New Topdown features in Intel Ice Lake
+New Topdown features in Intel Ice Lake and later p-core
 ======================================

 With Ice Lake CPUs the TopDown metrics are directly available as
 fixed counters and do not require generic counters. This allows
 to collect TopDown always in addition to other events.

-Using TopDown through RDPMC in applications on Intel Ice Lake
+Using TopDown through RDPMC in applications on Intel Ice Lake and later
p-core
 =============================================================

 For more fine grained measurements it can be useful to


> In the testing we're only testing per-task mode and not per-CPU:
> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/lib/perf/tests/test-evsel.c?h=perf-tools-next#n145

The event type is hard code to PERF_TYPE_HARDWARE, which is available
for both core. It doesn't matter which core the event is scheduled to.

If you plan to add a topdown event, the PERF_TYPE_RAW should be used,
which is the type for the p-core. If the app is scheduled on an e-core,
the event is disabled.

> It isn't clear to me how per-CPU would work given rescheduling, you
> could read a counter that doesn't match the mmap page unless you pin
> everything to just 1 CPU. Given the issues with rdpmc, perhaps it is
> safer in the documentation to cover cases without it.

As long as the scope of the RDPMC is clearly defined, I think it's OK
to add the usage in the documentation.

Thanks,
Kan