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

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Date:   Tue, 20 Jun 2017 14:36:25 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Dmitri Prokhorov <Dmitry.Prohorov@...el.com>,
        Valery Cherepennikov <valery.cherepennikov@...el.com>,
        David Carrillo-Cisneros <davidcc@...gle.com>,
        Stephane Eranian <eranian@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/n] perf/core: addressing 4x slowdown during
 per-process profiling of STREAM benchmark on Intel Xeon Phi

On Mon, Jun 19, 2017 at 11:31:59PM +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:
> >>+static int
> >>+perf_cpu_tree_iterate(struct rb_root *tree,
> >>+		perf_cpu_tree_callback_t callback, void *data)
> >>+{
> >>+	int ret = 0;
> >>+	struct rb_node *node;
> >>+	struct perf_event *event;
> >>+
> >>+	WARN_ON_ONCE(!tree);
> >>+
> >>+	for (node = rb_first(tree); node; node = rb_next(node)) {
> >>+		struct perf_event *node_event = container_of(node,
> >>+				struct perf_event, group_node);
> >>+
> >>+		list_for_each_entry(event, &node_event->group_list,
> >>+				group_list_entry) {
> >>+			ret = callback(event, data);
> >>+			if (ret)
> >>+				return ret;
> >>+		}
> >>+	}
> >>+
> >>+	return 0;
> >>  }
> >
> >If you need to iterate over every event, you can use the list that
> >threads the whole tree.
> 
> Could you please explain more on that?

In Peter's original suggestion, we'd use a threaded tree rather than a
tree of lists.

i.e. you'd have something like:

struct threaded_rb_node {
	struct rb_node   node;
	struct list_head head;
};

... with the tree and list covering all nodes, in the same order:

Tree:

     3
    / \
   /   \
  1     5
 / \   / \
0   2 4   6

List:

0 - 1 - 2 - 3 - 4 - 5 - 6

... that way you can search using the tree, and iterate using the list,
even when you wan to iterate over sub-lists.

Thanks,
Mark.