On 11/16/21 6:31 AM, Ian Rogers wrote:

On Sun, Nov 14, 2021 at 10:17 AM Ian Rogers [off-list ref] wrote:

quoted

On Sun, Nov 14, 2021 at 9:02 AM Jiri Olsa [off-list ref] wrote:

quoted

On Fri, Nov 12, 2021 at 11:15:48PM -0800, Ian Rogers wrote:

quoted

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?

quoted

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.

A different problem this fixes is if you have a group like
'{instructions,slots,topdown-fe-bound}' then slots still has to be the
group leader even though it is appearing before the topdown event. So
before the patch:

Hi Ian,
     Thanks for the patch series. We also have similar scenario in case
of powerpc for L2/L3 bus events, where it needs specific event as a
group leader. So the solution in this patch series will be helpful in
handling it.

Patch series looks good to me.

Reviewed-by: Kajol Jain<redacted>

Thanks,
Kajol Jain

$ perf stat -e '{instructions,slots,topdown-fe-bound}' -a -- sleep 2

 Performance counter stats for 'system wide':

     <not counted>      instructions
     <not counted>      slots
   <not supported>      topdown-fe-bound

       2.006410898 seconds time elapsed

After:

$ perf stat -e '{instructions,slots,topdown-fe-bound}' -a -- sleep 2

 Performance counter stats for 'system wide':

        2572632080      slots
         362537420      instructions
         742416906      topdown-fe-bound

       2.005875050 seconds time elapsed

Thanks,
Ian

quoted

quoted

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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

quoted

quoted

      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