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

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Date:   Tue, 30 Apr 2019 11:46:54 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ian Rogers <irogers@...gle.com>
Cc:     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



On 4/30/2019 5:08 AM, Peter Zijlstra wrote:
> 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.


The hierarchies is handled in the next patch as below.

But I once thought we only need to handle directly match. So it will be 
return immediately once a match found.
I believe we can fix it by simply remove the "return 0".

> +static int cgroup_visit_groups_merge(struct perf_event_groups *groups, int cpu,
> +				     int (*func)(struct perf_event *, void *, int (*)(struct perf_event *)),
> +				     void *data)
> +{
> +	struct sched_in_data *sid = data;
> +	struct cgroup_subsys_state *css;
> +	struct perf_cgroup *cgrp;
> +	struct perf_event *evt;
> +	u64 cgrp_id;
> +
> +	for (css = &sid->cpuctx->cgrp->css; css; css = css->parent) {
> +		/* root cgroup doesn't have events */
> +		if (css->id == 1)
> +			return 0;
> +
> +		cgrp = container_of(css, struct perf_cgroup, css);
> +		cgrp_id = *this_cpu_ptr(cgrp->cgrp_id);
> +		/* Only visit groups when the cgroup has events */
> +		if (cgrp_id) {
> +			evt = perf_event_groups_first_cgroup(groups, cpu, cgrp_id);
> +			while (evt) {
> +				if (func(evt, (void *)sid, pmu_filter_match))
> +					break;
> +				evt = perf_event_groups_next_cgroup(evt);
> +			}
> +			return 0;		<--- need to remove for hierarchies
> +		}
> +	}
> +
> +	return 0;
> +}

Thanks,
Kan