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

From: Alexey Budankov
Date: Fri Jun 16 2017 - 10:08:47 EST

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

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.