Re: [PATCH 04/10] perf stat: Add --exclude-guest option

[Namhyung Kim <namhyung@kernel.org>]

On Wed, Sep 25, 2024 at 09:49:14AM -0400, Liang, Kan wrote:
> 
> 
> On 2024-09-24 4:21 p.m., Namhyung Kim wrote:
> > On Mon, Sep 23, 2024 at 09:47:17AM +0100, James Clark wrote:
> >>
> >> On 06/09/2024 3:33 pm, Liang, Kan wrote:
> >>>
> >>> On 2024-09-05 4:24 p.m., Namhyung Kim wrote:
> >>>> This option is to support the old behavior of setting exclude_guest by
> >>>> default.  Now it doesn't set the bit so users want the old behavior can
> >>>> use this option.
> >>>>
> >>>>    $ perf stat true
> >>>>
> >>>>     Performance counter stats for 'true':
> >>>>
> >>>>                  0.86 msec task-clock:u                     #    0.443 CPUs utilized
> >>>>                     0      context-switches:u               #    0.000 /sec
> >>>>                     0      cpu-migrations:u                 #    0.000 /sec
> >>>>                    49      page-faults:u                    #   56.889 K/sec
> >>>>                   ...
> >>>>
> >>>>    $ perf stat --exclude-guest true
> >>>>
> >>>>     Performance counter stats for 'true':
> >>>>
> >>>>                  0.79 msec task-clock:Hu                    #    0.490 CPUs utilized
> >>>>                     0      context-switches:Hu              #    0.000 /sec
> >>>>                     0      cpu-migrations:Hu                #    0.000 /sec
> >>>>                    49      page-faults:Hu                   #   62.078 K/sec
> >>>>                   ...
> >>>>
> >>>> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> >>>> ---
> >>>>   tools/perf/Documentation/perf-stat.txt | 7 +++++++
> >>>>   tools/perf/builtin-stat.c              | 2 ++
> >>>>   2 files changed, 9 insertions(+)
> >>>>
> >>>> diff --git a/tools/perf/Documentation/perf-stat.txt b/tools/perf/Documentation/perf-stat.txt
> >>>> index 2bc06367248691dd..d28d8370a856598f 100644
> >>>> --- a/tools/perf/Documentation/perf-stat.txt
> >>>> +++ b/tools/perf/Documentation/perf-stat.txt
> >>>> @@ -382,6 +382,13 @@ color the metric's computed value.
> >>>>   Don't print output, warnings or messages. This is useful with perf stat
> >>>>   record below to only write data to the perf.data file.
> >>>> +--exclude-guest::
> >>>> +Don't count event in the guest mode.  It was the old behavior but the
> >>>> +default is changed to count guest events also.  Use this option if you
> >>>> +want the old behavior (host only).  Note that this option needs to be
> >>>> +before other events in case you added -e/--event option in the command
> >>>> +line.
> >>> I'm not sure if we really need this option. I think it may bring more
> >>> trouble than what we get.
> >>>
> >>> The name of the "--exclude-guest" sounds like a replacement of the event
> >>> modifier "H". But in fact, it's not. It should only affect the default.
> >>> It doesn't set the "H" for any events.
> > Well I think it's tricky but it'd set "H" modifier events after the
> > option.  But I have to agree that it can bring more troubles.
> 
> I may have miss-read something before. After some simple tests, yes, the
> "H" is applied with the option.

Ok.

> 
> Since there is a limit for the "--exclude-guest" option, can we print a
> warning if the option becomes invalid because of the order?

Well.. I'm thinking of removing this option for now.

> 
> > 
> >>> Except for the perf kvm user, I don't think there are many users which
> >>> care the exclude_guest. The behavior of the perf kvm is not changed. So
> >>> the option seems not that important. If we really want an option to
> >>> restore the old behavior, it's better to choose a better name and update
> >>> the description.
> > Personally I don't want to this option but just worried if there's a
> > case where exclude_guest is preferred.
> 
> The only case I can imagine is that, with the new vPMU passthrough
> introduced, some users may want to explicitly set the exclude_guest to
> avoid the fallback. But I'm not sure how useful it is for them.

Because of overhead?  They'll get exclude_guest eventually, right?

So I think I can drop this patch for now.  And consider this later when
we can find a real usecase.

Thanks,
Namhyung