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 <YZFBDI6oPMidX0KO@krava>
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From:   Ian Rogers <irogers@google.com>
Date:   Sun, 14 Nov 2021 10:17:25 -0800
Message-ID: <CAP-5=fWzzggdSv7bUm-+Ed6LfoUa1ce02n-bEOCqWWip4L8TLg@mail.gmail.com>
Subject: Re: [PATCH 2/2] perf parse-events: Architecture specific leader override
To:     Jiri Olsa <jolsa@redhat.com>
Cc:     Andi Kleen <ak@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>,
        John Garry <john.garry@huawei.com>,
        Kajol Jain <kjain@linux.ibm.com>,
        "Paul A . Clarke" <pc@us.ibm.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Riccardo Mancini <rickyman7@gmail.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
        eranian@google.com
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On Sun, Nov 14, 2021 at 9:02 AM Jiri Olsa <jolsa@redhat.com> wrote:
>
> On Fri, Nov 12, 2021 at 11:15:48PM -0800, Ian Rogers wrote:
> > Currently topdown events must appear after a slots event:
> >
> > $ perf stat -e '{slots,topdown-fe-bound}' /bin/true
> >
> >  Performance counter stats for '/bin/true':
> >
> >          3,183,090      slots
> >            986,133      topdown-fe-bound
> >
> > Reversing the events yields:
> >
> > $ perf stat -e '{topdown-fe-bound,slots}' /bin/true
> > Error:
> > The sys_perf_event_open() syscall returned with 22 (Invalid argument) for event (topdown-fe-bound).
>
> why is this actually failing?
>
> >
> > For metrics the order of events is determined by iterating over a
> > hashmap, and so slots isn't guaranteed to be first which can yield this
> > error.
> >
> > Change the set_leader in parse-events, called when a group is closed, so
> > that rather than always making the first event the leader, if the slots
> > event exists then it is made the leader. It is then moved to the head of
> > the evlist otherwise it won't be opened in the correct order.
> >
> > The result is:
> >
> > $ perf stat -e '{topdown-fe-bound,slots}' /bin/true
> >
> >  Performance counter stats for '/bin/true':
> >
> >          3,274,795      slots
> >          1,001,702      topdown-fe-bound
> >
> > A problem with this approach is the slots event is identified by name,
> > names can be overwritten like 'cpu/slots,name=foo/' and this causes the
> > leader change to fail.
>
> would it be better then to move this shuffle to the metric code directly,
> and make sure it only spits 'good' order of events?
>
> and keep "-e '{topdown-fe-bound,slots}'" to fail if user specifies that on
> the command line

It's an alternative to do that, but the people I spoke to preferred
having parse-events do this. I'm not sure what Andi's opinion is.
There is a general frustration about how brittle the slots event is,
and so this does make it less brittle.

> >
> > Signed-off-by: Ian Rogers <irogers@google.com>
> > ---
> >  tools/perf/arch/x86/util/evlist.c | 17 +++++++++++++++++
> >  tools/perf/util/evlist.h          |  1 +
> >  tools/perf/util/parse-events.c    | 16 +++++++++++-----
> >  tools/perf/util/parse-events.h    |  4 ++--
> >  tools/perf/util/parse-events.y    | 12 ++++++++----
> >  5 files changed, 39 insertions(+), 11 deletions(-)
> >
> > diff --git a/tools/perf/arch/x86/util/evlist.c b/tools/perf/arch/x86/util/evlist.c
> > index 0b0951030a2f..1624372b2da2 100644
> > --- a/tools/perf/arch/x86/util/evlist.c
> > +++ b/tools/perf/arch/x86/util/evlist.c
> > @@ -17,3 +17,20 @@ int arch_evlist__add_default_attrs(struct evlist *evlist)
> >       else
> >               return parse_events(evlist, TOPDOWN_L1_EVENTS, NULL);
> >  }
> > +
> > +struct evsel *arch_evlist__leader(struct list_head *list)
> > +{
> > +     struct evsel *evsel, *first;
> > +
> > +     first = list_entry(list->next, struct evsel, core.node);
> > +
> > +     if (!pmu_have_event("cpu", "slots"))
> > +             return first;
> > +
> > +     __evlist__for_each_entry(list, evsel) {
> > +             if (evsel->pmu_name && !strcmp(evsel->pmu_name, "cpu") &&
> > +                     evsel->name && strstr(evsel->name, "slots"))
> > +                     return evsel;
> > +     }
> > +     return first;
> > +}
> > diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
> > index 97bfb8d0be4f..993437ffe429 100644
> > --- a/tools/perf/util/evlist.h
> > +++ b/tools/perf/util/evlist.h
> > @@ -110,6 +110,7 @@ int __evlist__add_default_attrs(struct evlist *evlist,
> >       __evlist__add_default_attrs(evlist, array, ARRAY_SIZE(array))
> >
> >  int arch_evlist__add_default_attrs(struct evlist *evlist);
> > +struct evsel *arch_evlist__leader(struct list_head *list);
> >
> >  int evlist__add_dummy(struct evlist *evlist);
> >
> > diff --git a/tools/perf/util/parse-events.c b/tools/perf/util/parse-events.c
> > index 6308ba739d19..a2f4c086986f 100644
> > --- a/tools/perf/util/parse-events.c
> > +++ b/tools/perf/util/parse-events.c
> > @@ -1821,22 +1821,28 @@ parse_events__set_leader_for_uncore_aliase(char *name, struct list_head *list,
> >       return ret;
> >  }
> >
> > -void parse_events__set_leader(char *name, struct list_head *list,
> > -                           struct parse_events_state *parse_state)
> > +__weak struct evsel *arch_evlist__leader(struct list_head *list)
> > +{
> > +     return list_entry(list->next, struct evsel, core.node);
> > +}
> > +
> > +struct list_head *parse_events__set_leader(char *name, struct list_head *list,
> > +                                     struct parse_events_state *parse_state)
> >  {
> >       struct evsel *leader;
> >
> >       if (list_empty(list)) {
> >               WARN_ONCE(true, "WARNING: failed to set leader: empty list");
> > -             return;
> > +             return NULL;
> >       }
> >
> >       if (parse_events__set_leader_for_uncore_aliase(name, list, parse_state))
> > -             return;
> > +             return NULL;
> >
> > -     leader = list_entry(list->next, struct evsel, core.node);
> > +     leader = arch_evlist__leader(list);
> >       __perf_evlist__set_leader(list, &leader->core);
> >       leader->group_name = name ? strdup(name) : NULL;
> > +     return &leader->core.node;
> >  }
> >
> >  /* list_event is assumed to point to malloc'ed memory */
> > diff --git a/tools/perf/util/parse-events.h b/tools/perf/util/parse-events.h
> > index c7fc93f54577..8a6e6b4d5cbe 100644
> > --- a/tools/perf/util/parse-events.h
> > +++ b/tools/perf/util/parse-events.h
> > @@ -211,8 +211,8 @@ int parse_events_copy_term_list(struct list_head *old,
> >
> >  enum perf_pmu_event_symbol_type
> >  perf_pmu__parse_check(const char *name);
> > -void parse_events__set_leader(char *name, struct list_head *list,
> > -                           struct parse_events_state *parse_state);
> > +struct list_head *parse_events__set_leader(char *name, struct list_head *list,
> > +                                     struct parse_events_state *parse_state);
> >  void parse_events_update_lists(struct list_head *list_event,
> >                              struct list_head *list_all);
> >  void parse_events_evlist_error(struct parse_events_state *parse_state,
> > diff --git a/tools/perf/util/parse-events.y b/tools/perf/util/parse-events.y
> > index 174158982fae..5358c400f938 100644
> > --- a/tools/perf/util/parse-events.y
> > +++ b/tools/perf/util/parse-events.y
> > @@ -199,20 +199,24 @@ group_def
> >  group_def:
> >  PE_NAME '{' events '}'
> >  {
> > -     struct list_head *list = $3;
> > +     struct list_head *new_head, *list = $3;
> >
> >       inc_group_count(list, _parse_state);
> > -     parse_events__set_leader($1, list, _parse_state);
> > +     new_head = parse_events__set_leader($1, list, _parse_state);
> > +     if (new_head)
> > +             list_move(new_head, list);
>
> why not put these last 2 lines to parse_events__set_leader as well?

I can move that, but I'll try to motivate having it this way. The list
logic in Linux I find troublesome. In parse-events.y we have a list
head that is just a next, prev pointer and not full evsel in a list.
So for {topdown-fe-bound,slots} in parse-events.y we have a list of:

head -next-> evsel("topdown-fe-bound") -next-> evsel("slots")

But in __perf_evlist__set_leader the list doesn't have the head:

evsel("topdown-fe-bound") -next-> evsel("slots")

The list_move is changing the list to:

head -next-> evsel("slots") -next-> evsel("topdown-fe-bound")

which isn't possible in __perf_evlist__set_leader as the head has
gone. I felt having the list_move in parse_events__set_leader made it
look like the leader is being placed second rather than at the head.
So this was an attempt to make the code more intention revealing, but
that could also be done by some comments, it's just in
parse_events__set_leader we have the two styles of lists in play.

While I'm complaining, there are many brittle assumptions about evlist
that it may be worth adding some more assertions for. There is already
an assertion that the leader was opened before its siblings. There are
assumptions that the idx given to an evsel are incremental and that
for the leader 'idx - prev->idx' will yield the number of list
elements. My concern is for libperf users, it doesn't seem
unreasonable that they will want to manipulate the evlist but given
the assumptions they will likely find bugs - such as idx being used to
index an array and reordering breaking things. The metric code would
be a customer of reordering were it to need to move slots, but when I
experimented with this it easily broke things - which motivates doing
it in parse-events or more likely for the metrics, when generating the
parse-events string.

Thanks,
Ian


> >       free($1);
> >       $$ = list;
> >  }
> >  |
> >  '{' events '}'
> >  {
> > -     struct list_head *list = $2;
> > +     struct list_head *new_head, *list = $2;
> >
> >       inc_group_count(list, _parse_state);
> > -     parse_events__set_leader(NULL, list, _parse_state);
> > +     new_head = parse_events__set_leader(NULL, list, _parse_state);
> > +     if (new_head)
> > +             list_move(new_head, list);
>
> same
>
> thanks,
> jirka
>