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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Message-ID: <4611984d-0d77-1584-3011-768c80c261af@linux.intel.com>
Date:   Tue, 20 Jun 2017 20:10:06 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Mark Rutland <mark.rutland@....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 20.06.2017 19:37, Mark Rutland wrote:
> On Tue, Jun 20, 2017 at 06:22:56PM +0300, Alexey Budankov wrote:
>> On 20.06.2017 16:36, Mark Rutland wrote:
>>> 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;
>>> };
>>
>> Is this for every group leader?
> 
> Yes; *every* group leader would be directly in the threaded rb tree.

In this case the tree's key heeds to be something trickier than just 
event->cpu. To avoid that complication group_list is introduced. BTW, 
addressing perf_event_tree_delete issue doesn't look like a big change now:

static void
perf_cpu_tree_delete(struct rb_root *tree, struct perf_event *event)
{
	struct perf_event *next;

	WARN_ON_ONCE(!tree || !event);

	list_del_init(&event->group_entry);

	if (!RB_EMPTY_NODE(&event->group_node)) {
		if (!list_empty(&event->group_list)) {
			next = list_first_entry(&event->group_list,
					struct perf_event, group_entry);
			list_replace_init(&event->group_list,
					&next->group_list);
			rb_replace_node(&event->group_node,
					&next->group_node, tree);
		} else {
			rb_erase(&event->group_node, tree);
		}
		RB_CLEAR_NODE(&event->group_node);
	}
}

> 
>> Which objects does the head keep?
> 
> Sorry, I'm not sure how to answer that. Did the above clarify?
> 
> If not, could you rephrase the question?
> 
> Thanks,
> Mark.
>