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

[Alexey Budankov]

On 29.05.2017 15:03, Alexander Shishkin wrote:
> Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx> 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