Re: [PATCH]: perf/core: addressing 4x slowdown during per-process profiling of STREAM benchmark on Intel Xeon Phi

Alexey Budankov
2017-05-29T13:43:17Z

On 29.05.2017 15:03, Alexander Shishkin wrote:
> Alexey Budankov <alexey.budankov@linux.intel.com> writes:
>
> Here (above the function) you could include a comment describing what
> happens when this is called, locking considerations, etc.

I can put the short description from the initial thread message here.
Would it be sufficient?

>
>> +static int
>> +perf_cpu_tree_insert(struct rb_root *tree, struct perf_event *event)
>> +{
>> + struct rb_node **node;
>> + struct rb_node *parent;
>> +
>> + if (!tree || !event)
>> + return 0;
>
> I don't think this should be happening, should it? And either way you
> probably don't want to return 0 here, unless you're using !0 for
> success.

As you might notice already, currently return codes of the tree API are
not checked all other the implementation. I suggest replacing that int
error code by void and simplify the stuff.

>
>> +
>> + node = &tree->rb_node;
>> + parent = *node;
>> +
>> + while (*node) {
>> + struct perf_event *node_event = container_of(*node,
>> + struct perf_event, group_node);
>> +
>> + parent = *node;
>> +
>> + if (event->cpu < node_event->cpu) {
>> + node = &((*node)->rb_left);
>
> this would be the same as node = &parent->rb_left, right?

Please ask more. node is the leaf node and parent is the parent of the
node at the end of cycle. We need the both to insert a new node into a
tree.

>
>> + } else if (event->cpu > node_event->cpu) {
>> + node = &((*node)->rb_right);
>> + } else {
>> + list_add_tail(&event->group_list_entry,
>> + &node_event->group_list);
>
> So why is this better than simply having per-cpu event lists plus one
> for per-thread events?

Good question. Choice of data structure and layout depends on the
operations applied to the data so keeping groups as a tree simplifies
and improves the implementation in terms of scalability and performance.
Please ask more if any.

>
> Also,
>
>> + return 2;
>
> 2?

Answered above.

>
>> + }
>> + }
>> +
>> + list_add_tail(&event->group_list_entry, &event->group_list);
>> +
>> + rb_link_node(&event->group_node, parent, node);
>> + rb_insert_color(&event->group_node, tree);
>> +
>> + return 1;
>
> Oh, you are using !0 for success. I guess it's a good thing you're not
> actually checking its return code at the call site.

Answered above.

>
> Regards,
> --
> Alex
>