Re: [PATCH v2] perf: Skip and warn on unknown format 'configN' attrs

[Rob Herring <robh@kernel.org>]

On Mon, Sep 12, 2022 at 5:35 AM Leo Yan <leo.yan@linaro.org> wrote:
>
> Hi Rob,
>
> On Fri, Sep 09, 2022 at 03:45:09PM -0500, Rob Herring wrote:
> > If the kernel exposes a new perf_event_attr field in a format attr, perf
> > will return an error stating the specified PMU can't be found. For
> > example, a format attr with 'config3:0-63' causes an error as config3 is
> > unknown to perf. This causes a compatibility issue between a newer
> > kernel with older perf tool.
> >
> > Before this change with a kernel adding 'config3' I get:
> >
> > $ perf record -e arm_spe// -- true
> > event syntax error: 'arm_spe//'
> >                      \___ Cannot find PMU `arm_spe'. Missing kernel support?
> > Run 'perf list' for a list of valid events
> >
> >  Usage: perf record [<options>] [<command>]
> >     or: perf record [<options>] -- <command> [<options>]
> >
> >     -e, --event <event>   event selector. use 'perf list' to list
> > available events
> >
> > After this change, I get:
> >
> > $ perf record -e arm_spe// -- true
> > WARNING: format 'inv_event_filter' requires 'config3' which is not supported by this version of perf!
> > [ perf record: Woken up 2 times to write data ]
> > [ perf record: Captured and wrote 0.091 MB perf.data ]
> >
> > To support unknown configN formats, rework the YACC implementation to
> > pass any config[0-9]+ format to perf_pmu__new_format() to handle with a
> > warning.
> >
> > Note that the user will get the warning if *any* PMU has an unsupported
> > format attr even if that PMU isn't used. This is because perf tool scans
> > all the PMUs.
>
> I think essentially you want to provide a bug fixing and the fixing can
> be back ported on long term supported kernels?

Yes, certainly. I forgot to note that on this version.

> If this is the case,
> it's good to add a fixes tag like below?
>
> Fixes: cd82a32e9924 ("perf tools: Add perf pmu object to access pmu format definition")

Probably not too important given this is from 2012.

>
> > Signed-off-by: Rob Herring <robh@kernel.org>
> > ---
> > v2:
> >  - Rework YACC code to handle configN formats in C code
> >  - Add a warning when an unknown configN attr is found
> >
> > v1: https://lore.kernel.org/all/20220901184709.2179309-1-robh@kernel.org/
> > ---
> >  tools/perf/util/pmu.c |  6 ++++++
> >  tools/perf/util/pmu.l |  2 --
> >  tools/perf/util/pmu.y | 15 ++++-----------
> >  3 files changed, 10 insertions(+), 13 deletions(-)
> >
> > diff --git a/tools/perf/util/pmu.c b/tools/perf/util/pmu.c
> > index 89655d53117a..6757db7d559c 100644
> > --- a/tools/perf/util/pmu.c
> > +++ b/tools/perf/util/pmu.c
> > @@ -1475,6 +1475,12 @@ int perf_pmu__new_format(struct list_head *list, char *name,
> >  {
> >       struct perf_pmu_format *format;
> >
> > +     if (config > PERF_PMU_FORMAT_VALUE_CONFIG2) {
>
> It's good to add a new macro PERF_PMU_FORMAT_VALUE_CONFIG_END in
> util/pmu.h. Then at here we can check the condition:

Sure.

>
>        if (config >= PERF_PMU_FORMAT_VALUE_CONFIG_END) {
>
> > +             pr_warning("WARNING: format '%s' requires 'config%d' which is not supported by this version of perf!\n",
> > +                        name, config);
>
> ... so at here we can print log like:
>
>        pr_warning("WARNING: format '%s' requires 'config%d' which is not "
>                   "supported by this version of perf (maximum config%d)!\n",
>                   name, config, PERF_PMU_FORMAT_VALUE_CONFIG_END - 1);

I was trying to keep it to one line and given configN isn't expanded
frequently it should be simple enough to figure out what version you
need.

>
> The rest of this patch is fine for me.
>
> As a side topic, I know you want to support the SPEv1.2 feature with
> config3, seems to me a complete patch set series should include the
> changes for supporting config3 as well.  This can give more clear view
> for how to fix incompatibility issue between old and new kernels, and
> how to support config3 in the latest kernel, but it's fine for me if
> you want to split into two patch sets.

I've sent out the SPE kernel support separately. I was told by Arnaldo
to split perf tool and kernel side changes.

Rob