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

[Alexey Budankov <alexey.budankov@linux.intel.com>]

On 30.05.2017 11:29, Alexander Shishkin wrote:
> Alexey Budankov <alexey.budankov@linux.intel.com> writes:
> 
>> 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?
> 
> 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
>