Re: Invalid event (cycles:pp) in per-thread mode, enable system wide with '-a'.

From: Ravi Bangoria
Date: Thu Oct 06 2022 - 23:56:52 EST

+cc: PeterZ

>>>>> +Ravi who may be able to say if there are any issues with the precise
>>>>> sampling on AMD.
>>>> Afaik cvcles:pp will use IBS but it doesn't support per-task profiling
>>>> since it has no task context. Ravi is working on it..
>>> Right.
>> Cool, thanks for working on this Ravi.
>> I'm not sure yet whether I may replace the kernel on my corporate
>> provided workstation, so I'm not sure yet I can help test that patch.
>> Can you confirm that
>> $ perf record -e cycles:pp --freq=128 --call-graph lbr -- <command to profile>
>> works with just that patch applied? Or is there more work required?
>> What is the status of that patch?
>> For context, we had difficulty upstreaming support for instrumentation
>> based profile guided optimizations in the Linux kernel.
>> We'd like to be able to use either instrumentation or sampling to
>> optimize our builds. The major barrier to sample based approaches are
>> architecture / micro architecture issues with sample based profile
>> data collection, and bitrot of data processing utilities.
> On existing AMD Zen2, Zen3 the following cmdline:
> $ perf record -e cycles:pp --freq=128 --call-graph lbr -- <command to profile>
> does not work. I see two reasons:
> 1. cycles:pp is likely converted into IBS op in cycle mode.
> Current kernels do not support IBS in per-thread mode.
> This is purely a kernel limitation

Right, it's purely a kernel limitation. And below simple patch on top
of event-context rewrite patch[1] should be sufficient to make cycles:pp
working in per-process mode on AMD Zen.

diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
index c251bc44c088..de01b5d27e40 100644
--- a/arch/x86/events/amd/ibs.c
+++ b/arch/x86/events/amd/ibs.c
@@ -665,7 +665,7 @@ static struct perf_ibs perf_ibs_fetch = {

static struct perf_ibs perf_ibs_op = {
.pmu = {
- .task_ctx_nr = perf_invalid_context,
+ .task_ctx_nr = perf_hw_context,

.event_init = perf_ibs_init,
.add = perf_ibs_add,