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

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Message-ID: <20190430090847.GM2623@hirez.programming.kicks-ass.net>
Date:   Tue, 30 Apr 2019 11:08:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ian Rogers <irogers@...gle.com>
Cc:     kan.liang@...ux.intel.com, tglx@...utronix.de, mingo@...hat.com,
        linux-kernel@...r.kernel.org,
        Stephane Eranian <eranian@...gle.com>, tj@...nel.org,
        ak@...ux.intel.com
Subject: Re: [PATCH 3/4] perf cgroup: Add cgroup ID as a key of RB tree


A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?

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.