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

[Peter Zijlstra]

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.