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

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Message-ID: <85822c3c-2254-52cc-e6b1-9c89adb63771@amd.com>
Date:   Fri, 7 Oct 2022 09:26:30 +0530
From:   Ravi Bangoria <ravi.bangoria@....com>
To:     Stephane Eranian <eranian@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Slade Watkins <srw@...dewatkins.net>,
        linux-perf-users <linux-perf-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ian Rogers <irogers@...gle.com>,
        Namhyung Kim <namhyung@...il.com>,
        Kees Cook <keescook@...omium.org>, sandipan.das@....com,
        Bill Wendling <morbo@...gle.com>,
        clang-built-linux <llvm@...ts.linux.dev>,
        Yonghong Song <yhs@...com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: Invalid event (cycles:pp) in per-thread mode, enable system wide
 with '-a'.

+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.
>>> https://lore.kernel.org/lkml/20220829113347.295-1-ravi.bangoria@amd.com
>>
>> 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.
>> https://lore.kernel.org/lkml/CAHk-=whqCT0BeqBQhW8D-YoLLgp_eFY=8Y=9ieREM5xx0ef08w@mail.gmail.com/
>> 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.
>> https://github.com/google/autofdo/issues/144
> 
> 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,
---

[1]: https://lore.kernel.org/lkml/20220829113347.295-1-ravi.bangoria@amd.com