Re: [PATCH v3 1/n] perf/core: addressing 4x slowdown during per-process profiling of STREAM benchmark on Intel Xeon Phi

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

On 16.06.2017 12:09, Mark Rutland wrote:
> On Fri, Jun 16, 2017 at 01:10:10AM +0300, Alexey Budankov wrote:
>> On 15.06.2017 22:56, Mark Rutland wrote:
>>> On Thu, Jun 15, 2017 at 08:41:42PM +0300, Alexey Budankov wrote:
>>>> This series of patches continues v2 and addresses captured comments.
> 
>>>> Specifically this patch replaces pinned_groups and flexible_groups
>>>> lists of perf_event_context by red-black cpu indexed trees avoiding
>>>> data structures duplication and introducing possibility to iterate
>>>> event groups for a specific CPU only.
>>>
>>> If you use --per-thread, I take it the overhead is significantly
>>> lowered?
>>
>> Please ask more.
> 
> IIUC, you're seeing the slowdown when using perf record, correct?

Correct. Specifically in per-process mode - without -a option.

> 
> There's a --per-thread option to ask perf record to not duplicate the
> event per-cpu.
> 
> If you use that, what amount of slowdown do you see?
> 
> It might be preferable to not open task-bound per-cpu events on systems
> with large cpu counts, and it would be good to know what the trade-off
> looks like for this case.
> 
>>>> +static void
>>>> +perf_cpu_tree_insert(struct rb_root *tree, struct perf_event *event)
>>>> +{
>>>> +	struct rb_node **node;
>>>> +	struct rb_node *parent;
>>>> +
>>>> +	WARN_ON_ONCE(!tree || !event);
>>>> +
>>>> +	node = &tree->rb_node;
>>>> +	parent = *node;
>>>
>>> The first iteration of the loop handles this, so it can go.
>>
>> If tree is empty parent will be uninitialized what is harmful.
> 
> Sorry; my bad.
> 
> Thanks,
> Mark.
>