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

[Alexey Budankov]

On 30.05.2017 11:29, Alexander Shishkin wrote:
> Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx> writes:
>
> > 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?
>
> Sure, this is where API descriptions fit better than in commit messages.
>
> > > > +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.
>
> Your call. But I'd still either drop the redundant checks or wrap them
> in WARN_ON_ONCE().

Ok. WARN_ON_ONCE() then.

> > > > +
> > > > +	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?

Yes, that is right.

> > Please ask more.
>
> Side note: between commit message, comments and the actual code, in an
> ideal situation one doesn't have to 'ask' anything, because everything
> is already clear. Not the case here.
>
> > 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.
>
> Not sure I understand. You'd still have both.
>
> > > > +		} 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.
>
> Please be more specific on how scalability and performance are
> improved. In general, try to avoid vagues statements like "this is
> better for performance".

Accepted. Peter already provided more specifics on this. Thanks.

> Thanks,
> --
> Alex