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

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Message-ID: <87k24zzx7s.fsf@ashishki-desk.ger.corp.intel.com>
Date:   Mon, 29 May 2017 15:03:35 +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:

Here (above the function) you could include a comment describing what
happens when this is called, locking considerations, etc.

> +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.

> +
> +	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?

> +		} 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?

Also,

> +			return 2;

2?

> +		}
> +	}
> +
> +	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.

Regards,
--
Alex