Re: [PATCH 0/4] perf intel-pt: Fix the pipe mode (v1)

[Namhyung Kim <namhyung@kernel.org>]

On Mon, Jan 30, 2023 at 2:56 AM James Clark <james.clark@arm.com> wrote:
>
>
>
> On 27/01/2023 23:08, Namhyung Kim wrote:
> > Hi James,
> >
> > On Fri, Jan 27, 2023 at 6:42 AM James Clark <james.clark@arm.com> wrote:
> >>
> >>
> >>
> >> On 27/01/2023 07:22, Adrian Hunter wrote:
> >>> On 27/01/23 02:19, Namhyung Kim wrote:
> >>>> Hello,
> >>>>
> >>>> I found some problems in Intel-PT and auxtrace in general with pipe.
> >>>> In the past it used to work with pipe, but recent code fails.
> >>>
> >>> Pipe mode is a problem for Intel PT and possibly other auxtrace users.
> >>
> >> Just some info from my side: For Arm Coresight we ended up deprecating
> >> pipe mode, then not supporting it altogether. First was when we added an
> >> optional step to peek through all of the data to help with an edge case.
> >> Then we added a requirement to receive a HW_ID packet before decoding
> >> which necessitated the peek. You can't peek in pipe mode because you
> >> need to be able to seek, so it's not supported at all anymore.
> >>
> >> For Arm SPE I never tested it with piped data. I suppose I could add a
> >> test at some point, but I don't really see the usecase.
> >
> > Yeah, it'd be great if we can have a test for Arm SPE.
> >
>
> Ok thanks I will put it on the list of things to do.
>
> > Anyway, my work env (Google) requires the pipe mode due to the
> > restriction in disk usage.  Without the pipe support, it's not possible
> > to run `perf record` in production.
> >
>
> Makes sense. Unfortunately at the moment with Coresight, because of the
> lack of appropriate timestamps we're waiting for the end of the file
> before starting decoding. So you're not really any better off using
> piped mode, unless you have a lot more memory than disk space?
>
> Since this commit [1] and Arm v8.4 we can actually start making use of
> the timestamps and do a streaming decode again. So I will also add it to
> the list to look into that for Coresight again. Are you using an old
> version of Perf or not using Coresight at all? I know Denis at Google is
> using Coresight, but only with files rather than pipes.

I'm not aware of usage of Coresight yet in my boundary, but others
may be using it.

>
> One other thing, have you used the --switch-output mode to perf record
> before? I would have said it would give you some of the benefits of
> piped mode, but is more likely to work with Coresight. But last time I
> checked it's not working either. Not very helpful I know, but something
> to keep in mind.

I don't think it'd work because it still occupies the same space.
So far, the pipe mode worked well but I think it needs some
more improvements.

Anyway, thanks for your suggestion and review!

Thanks,
Namhyung