Re: [PATCH v5 6/8] perf report: Add --latency flag

Namhyung Kim
2025-02-11T17:42:30Z

On Tue, Feb 11, 2025 at 09:42:16AM +0100, Dmitry Vyukov wrote:
> On Tue, 11 Feb 2025 at 02:02, Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > On Fri, Feb 07, 2025 at 08:23:58AM +0100, Dmitry Vyukov wrote:
> > > On Fri, 7 Feb 2025 at 04:44, Namhyung Kim <namhyung@kernel.org> wrote:
[SNIP]
> > > > > @@ -3547,10 +3549,15 @@ static int __hpp_dimension__add_output(struct perf_hpp_list *list,
> > > > > return 0;
> > > > > }
> > > > >
> > > > > -int hpp_dimension__add_output(unsigned col)
> > > > > +int hpp_dimension__add_output(unsigned col, bool implicit)
> > > > > {
> > > > > + struct hpp_dimension *hd;
> > > > > +
> > > > > BUG_ON(col >= PERF_HPP__MAX_INDEX);
> > > > > - return __hpp_dimension__add_output(&perf_hpp_list, &hpp_sort_dimensions[col]);
> > > > > + hd = &hpp_sort_dimensions[col];
> > > > > + if (implicit && !hd->was_taken)
> > > > > + return 0;
> > > >
> > > > I don't think you need these implicit and was_taken things.
> > > > Just removing from the sort list when it's unregistered seems to work.
> > > >
> > > > ---8<---
> > > > @@ -685,6 +685,7 @@ void perf_hpp_list__prepend_sort_field(struct perf_hpp_list *list,
> > > > static void perf_hpp__column_unregister(struct perf_hpp_fmt *format)
> > > > {
> > > > list_del_init(&format->list);
> > > > + list_del_init(&format->sort_list);
> > > > fmt_free(format);
> > > > }
> > > >
> > > > ---8<---
> > >
> > > It merely suppresses the warning, but does not work the same way. See
> > > this for details:
> > > https://lore.kernel.org/all/CACT4Y+ZREdDL7a+DMKGFGae1ZjX1C8uNRwCGF0c8iUJtTTq0Lw@mail.gmail.com/
> >
> > But I think it's better to pass --latency option rather than adding it
> > to -s option. If you really want to have specific output fields, then
> > please use -F latency,sym instead.
> >
> > Also I've realized that it should add one sort key in setup_overhead()
> > to support hierarchy mode properly. Something like this?
> >
> > Thanks,
> > Namhyung
> >
> >
> > ---8<---
> > diff --git a/tools/perf/util/sort.c b/tools/perf/util/sort.c
> > index 2b6023de7a53ae2e..329c2e9bbc69a725 100644
> > --- a/tools/perf/util/sort.c
> > +++ b/tools/perf/util/sort.c
> > @@ -3817,22 +3817,15 @@ static char *setup_overhead(char *keys)
> > return keys;
> >
> > if (symbol_conf.prefer_latency) {
> > - keys = prefix_if_not_in("overhead", keys);
> > - keys = prefix_if_not_in("latency", keys);
> > - if (symbol_conf.cumulate_callchain) {
> > - keys = prefix_if_not_in("overhead_children", keys);
> > + if (symbol_conf.cumulate_callchain)
> > keys = prefix_if_not_in("latency_children", keys);
> > - }
> > - } else if (!keys || (!strstr(keys, "overhead") &&
> > - !strstr(keys, "latency"))) {
> > - if (symbol_conf.enable_latency)
> > + else
> > keys = prefix_if_not_in("latency", keys);
> > - keys = prefix_if_not_in("overhead", keys);
> > - if (symbol_conf.cumulate_callchain) {
> > - if (symbol_conf.enable_latency)
> > - keys = prefix_if_not_in("latency_children", keys);
> > + } else {
> > + if (symbol_conf.cumulate_callchain)
> > keys = prefix_if_not_in("overhead_children", keys);
> > - }
> > + else
> > + keys = prefix_if_not_in("overhead", keys);
> > }
> >
> > return keys;
>
>
> Have I decoded the patch correctly?
>
> if (symbol_conf.prefer_latency) {
> if (symbol_conf.cumulate_callchain)
> keys = prefix_if_not_in("latency_children", keys);
> else
> keys = prefix_if_not_in("latency", keys);
> } else {
> if (symbol_conf.cumulate_callchain)
> keys = prefix_if_not_in("overhead_children", keys);
> else
> keys = prefix_if_not_in("overhead", keys);
> }
>

Yep, that's correct.

> If I decoded the patch correctly, it's not what we want.
>
> For the default prefer_latency case we also want to add overhead, that
> was intentional for the --latency preset. It does not harm, and allows
> to see/compare differences in latency and overhead.
> Again, if a user wants something custom, there is no way to second
> guess all possible intentions. For non-default cases, we just let the
> user say what exactly they want, and we will follow that.
>
> "latency" should be added even if cumulate_callchain.

Please note that it just sets the sort key - which column you want to
order the result. The output fields for overhead and children will be
added in perf_hpp__init() if you remove the 'was_taken' logic. So I
think this change will have the same output with that.

>
> For the !prefer_latency case, we don't want to mess with
> overhead/latency fields if the user specified any of them explicitly.
> Otherwise this convenience part gets in the user's way and does not
> allow them to do what they want. User says "I want X" and perf says
> "screw you, I will give you Y instead, and won't allow you to possibly
> do X".

That's what -F option does. The -s option used to specify how to group
the histogram entries and it will add 'overhead' (and/or 'latency') if
it's not even requested. So I think it's ok to add more output column
when -s option is used.

But unfortunately, using -F and -s together is confusing and change the
meaning of -s option - it now says how it sort the result.

>
> And see above: -F does not work with --hierarchy, so this part is unskippable.

Yep, but I mean it fixes --hierarchy and --latency. I'm thinking of a
way to support -F and --hierarchy in general.

Thanks,
Namhyung