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:   Tue, 30 May 2017 11:29:00 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Alexey Budankov <alexey.budankov@...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

Alexey Budankov <alexey.budankov@...ux.intel.com> writes:

> 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?

Sure, this is where API descriptions fit better than in commit messages.

>
>> 
>>> +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.

Your call. But I'd still either drop the redundant checks or wrap them
in WARN_ON_ONCE().

>
>> 
>>> +
>>> +	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.

Side note: between commit message, comments and the actual code, in an
ideal situation one doesn't have to 'ask' anything, because everything
is already clear. Not the case here.

> 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.

Not sure I understand. You'd still have both.

>
>> 
>>> +		} 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.

Please be more specific on how scalability and performance are
improved. In general, try to avoid vagues statements like "this is
better for performance".

Thanks,
--
Alex