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

Alexey Budankov
2017-06-15T17:44:39Z

On 14.06.2017 13:07, Alexey Budankov wrote:
> 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.

addressed in v3.

>>
>>>
>>>>
>>>>> +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.

addressed in v3.

>
>>
>>>
>>>>
>>>>> +
>>>>> + 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.

addressed in v3.

>
>>>
>>> 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
>>
>