linux-kernel - Re: [PATCH v3] perf evsel: Fix missing exclude

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Message-ID: <YYL5FaKQ1ZvGwd08@kernel.org>
Date:   Wed, 3 Nov 2021 18:03:17 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     Jiri Olsa <jolsa@...hat.com>, Namhyung Kim <namhyung@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>
Subject: Re: [PATCH v3] perf evsel: Fix missing exclude_{host,guest} setting

Em Wed, Nov 03, 2021 at 10:35:04AM -0700, Stephane Eranian escreveu:
> On Wed, Nov 3, 2021 at 4:32 AM Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> > Em Wed, Nov 03, 2021 at 12:44:12AM -0700, Stephane Eranian escreveu:
> > > On Wed, Nov 3, 2021 at 12:24 AM Jiri Olsa <jolsa@...hat.com> wrote:
> > > > > If the pmu doesn't support host/guest filtering, pmu/bla1/G
> > > > > may count something.  Not sure if it's better to error out.
> > > > > But the cycles:G and instructions:G should result in 0
> > > > > in case there's no VM running.

> > > > hm, I think if pmu doesn't support host/guest filtering then
> > > > I think 'pmu/bla1/G' should error, no? better no number than
> > > > bad number

> > > Yes, it should in my opinion.

> > Yeah, I thought about this yesterday (holiday here).

> Otherwise you create the illusion that you are monitoring in guest
> mode when you are not.

> The question is: how can the tool know which modifiers are supported
> per pmu model?

As things stand kernel-wise, we should just do capability querying, i.e.
if the user asks for a feature not available for a specific PMU, we
should refuse and provide a helpful error message to the user.

If the PMUs in the kernel had some kind of mask that stated what of the
'struct perf_event_attr' selectable features are supported, then we
would just be able to avoid bothering the kernel asking for unsupported
stuff.

Just for exclude_guest we don't even need to have
evsel->pmu->missing_features.exclude_guest, as this is a hard error, no
point in caching previous capability queries.

- Arnaldo