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

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Date:   Mon, 29 May 2017 16:43:09 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     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>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH]: perf/core: addressing 4x slowdown during per-process
 profiling of STREAM benchmark on Intel Xeon Phi

On 29.05.2017 15:03, Alexander Shishkin wrote:
> Alexey Budankov <alexey.budankov@...ux.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?

> 
>> +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.

> 
>> +
>> +	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?

Please ask more. 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.

> 
>> +		} 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.

> 
> Also,
> 
>> +			return 2;
> 
> 2?

Answered above.

> 
>> +		}
>> +	}
>> +
>> +	list_add_tail(&event->group_list_entry, &event->group_list);
>> +
>> +	rb_link_node(&event->group_node, parent, node);
>> +	rb_insert_color(&event->group_node, tree);
>> +
>> +	return 1;
> 
> Oh, you are using !0 for success. I guess it's a good thing you're not
> actually checking its return code at the call site.

Answered above.

> 
> Regards,
> --
> Alex
>