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

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Date:   Tue, 11 Oct 2022 14:38:21 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Stephane Eranian <eranian@...gle.com>,
        Ravi Bangoria <ravi.bangoria@....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>
Subject: Re: Invalid event (cycles:pp) in per-thread mode, enable system wide
 with '-a'.

On Wed, Oct 5, 2022 at 3:50 PM Stephane Eranian <eranian@...gle.com> wrote:
>
> On Wed, Oct 5, 2022 at 2:56 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> >
> > + Stephane, Kees, Sandipan, Bill, ClangBuiltLinux mailing list, Yonghong
> > https://www.spinics.net/lists/linux-perf-users/msg23103.html
> > starts the thread, for context.
> >
> > On Thu, Sep 29, 2022 at 9:32 PM Ravi Bangoria <ravi.bangoria@....com> wrote:
> > >
> > > On 30-Sep-22 9:56 AM, Namhyung Kim wrote:
> > > > Hello,
> > > >
> > > > On Thu, Sep 29, 2022 at 8:49 PM Ian Rogers <irogers@...gle.com> wrote:
> > > >>
> > > >> On Thu, Sep 29, 2022 at 3:10 PM Slade Watkins <srw@...dewatkins.net> wrote:
> > > >>>
> > > >>> Hey Nick,
> > > >>>
> > > >>>> On Sep 29, 2022, at 5:54 PM, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> > > >>>>
> > > >>>> I remember hearing rumblings about issues with zen 2, LBR, vs zen 3.
> > > >>>> Is this a known issue, or am I holding it wrong?
> > > >>>
> > > >>> Hm… I also remember this. I have a Zen 2 based system that I can do testing on, so I will do so when I’m able.
> > > >>>
> > > >>> If I discover something of note, I’ll get back to you.
> > > >>>
> > > >>> Cheers,
> > > >>> -srw
> > > >>>
> > > >>
> > > >> LBR isn't yet supported for Zen but is coming:
> > > >> https://lore.kernel.org/lkml/166155216401.401.5809694678609694438.tip-bot2@tip-bot2/
> > > >> I'd recommend frame-pointers.
> >
> > Having to recompile is less than ideal for my workflow.  I have added a note to
> > https://github.com/ClangBuiltLinux/profiling/tree/main/perf#errors
> > Please let me know how I might improve the documentation.
> >
> > > >>
> > > >> +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

Sounds like Ravi has a diff that might work?
https://lore.kernel.org/linux-perf-users/85822c3c-2254-52cc-e6b1-9c89adb63771@amd.com/

>
> 2. call-graph lbr is not supported on AMD because they do
>    not have LBR and therefore no LBR callstack mode
>
> The best way to get what you want here today on AMD Zen2 and Zen3:
>
>    $ perf record -e cycles --freq=128 -g -- <command to profile>

So without recompiling to explicitly re-enable frame pointers, code
I'm looking to profile is built with -gmlt, so I get symbols, but I
can't observe callers.

Sounds like I'd need to rebuild with -fno-omit-frame-pointers which is
more painful than my prior LBR based workflow.

>
> On AMD Zen3, there is a precursor to LBR with Branch Sampling (BRS),
> and you can use it to sample taken branches but not for callstacks. I
> mention the cmdline here for reference:
>
> $ perf record -e cpu/branch-brs/ -c 1000037  -b  -- <command to profile>
>
> Note that AMD Zen3 BRS is enough to get the autoFDO usage of an
> LBR working as per the cmdline above.

Interesting, but I'm stuck with a Zen2 box for a couple years now
(corporate workstation).  This pretty much blows up all of the
profiling work I was doing, and any hope I had of contributing towards
building the kernel with AutoFDO profiles until this works on my box
with the kernel it has.

>
> Hope this helps.



-- 
Thanks,
~Nick Desaulniers