linux-kernel - Re: [PATCH 2/7] perf/cgroup: order events in RB tree by cgroup id

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Message-ID: <20190708154528.GD3419@hirez.programming.kicks-ass.net>
Date:   Mon, 8 Jul 2019 17:45:28 +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 2/7] perf/cgroup: order events in RB tree by cgroup id

On Mon, Jul 01, 2019 at 11:59:50PM -0700, Ian Rogers wrote:
> +perf_event_groups_first(struct perf_event_groups *groups, int cpu,
> +			struct cgroup *cgrp)
>  {
>  	struct perf_event *node_event = NULL, *match = NULL;
>  	struct rb_node *node = groups->tree.rb_node;
> +#ifdef CONFIG_CGROUP_PERF
> +	int node_cgrp_id, cgrp_id = 0;
> +
> +	if (cgrp)
> +		cgrp_id = cgrp->id;
> +#endif

Is 0 ever a valid cgroup.id ? If so, should we perhaps use -1 to denote
'none' ? Ether way around a little comment here couldn't hurt, saves one
from digging into the cgroup code.