linux-kernel - Re: [PATCH 3/7] perf: order iterators for visit_groups

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Message-ID: <20190708163632.GH3419@hirez.programming.kicks-ass.net>
Date:   Mon, 8 Jul 2019 18:36:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org,
        Kan Liang <kan.liang@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 3/7] perf: order iterators for visit_groups_merge into a
 min-heap

On Mon, Jul 01, 2019 at 11:59:51PM -0700, Ian Rogers wrote:

> +/* Sift the perf_event at pos down the heap. */
> +static void min_heapify(struct perf_event_heap *heap, int pos)
> +{
> +	int left_child, right_child;
> +
> +	while (pos > heap->num_elements / 2) {

Did that want to be 'pos < head->num_elements / 2' ?

> +		left_child = pos * 2;
> +		right_child = pos * 2 + 1;
> +		if (heap->storage[pos]->group_index >
> +		    heap->storage[left_child]->group_index) {
> +			min_heap_swap(heap, pos, left_child);
> +			pos = left_child;
> +		} else if (heap->storage[pos]->group_index >
> +			   heap->storage[right_child]->group_index) {
> +			min_heap_swap(heap, pos, right_child);
> +			pos = right_child;
> +		} else {
> +			break;
> +		}
> +	}
> +}
> +
> +/* Floyd's approach to heapification that is O(n). */
> +static void min_heapify_all(struct perf_event_heap *heap)
> +{
> +	int i;
> +
> +	for (i = heap->num_elements / 2; i > 0; i--)
> +		min_heapify(heap, i);
> +}

Otherwise I don't actually see it do anything at all. Also, when pos >
nr/2, then pos * 2 is silly.