Re: [PATCH 3/4] perf cgroup: Add cgroup ID as a key of RB tree

[Peter Zijlstra <peterz@infradead.org>]

A: Because it messes up the order in which people normally read text.
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A: Top-posting.
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On Mon, Apr 29, 2019 at 04:02:33PM -0700, Ian Rogers wrote:
> This is very interesting. How does the code handle cgroup hierarchies?
> For example, if we have:
> 
> cgroup0 is the cgroup root
> cgroup1 whose parent is cgroup0
> cgroup2 whose parent is cgroup1
> 
> we have task0 running in cgroup0, task1 in cgroup1, task2 in cgroup2
> and then a perf command line like:
> perf stat -e cycles,cycles,cycles -G cgroup0,cgroup1,cgroup2 --no-merge sleep 10
> 
> we expected 3 cycles counts:
>  - for cgroup0 including task2, task1 and task0
>  - for cgroup1 including task2 and task1
>  - for cgroup2 just including task2
> 
> It looks as though:
> +       if (next && (next->cpu == event->cpu) && (next->cgrp_id ==
> event->cgrp_id))
> 
> will mean that events will only consider cgroups that directly match
> the cgroup of the event. Ie we'd get 3 cycles counts of:
>  - for cgroup0 just task0
>  - for cgroup1 just task1
>  - for cgroup2 just task2

Yeah, I think you're right; the proposed code doesn't capture the
hierarchy thing at all.