Re: [PATCH] perf: update perf_cgroup time for ancestor cgroup(s)

[Song Liu]

> On Mar 12, 2018, at 5:38 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> On Sun, Mar 11, 2018 at 10:32:44PM -0700, Song Liu wrote:
>> When a perf_event is attached to parent cgroup, it should count events
>> for all children cgroups:
>> 
>> parent_group   <---- perf_event
>>   \
>>    - child_group  <---- process(es)
>> 
>> However, in our tests, we found this perf_event cannot report reliable
>> results. This is because perf_event->cgrp and cpuctx->cgrp are not
>> identical, thus perf_event->cgrp are not updated properly.
> 
> It might help now and in for our older selves, if you could provide a
> simple reproducer for this.

I will add a repro to v2. 

> 
>> Signed-off-by: Song Liu <songliubraving@xxxxxx>
>> Reported-by: Ephraim Park <ephiepark@xxxxxx>
>> ---
>> kernel/events/core.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++-
>> 1 file changed, 67 insertions(+), 1 deletion(-)
>> 
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 5789810..623d38f 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
> 
>> @@ -766,6 +821,17 @@ perf_cgroup_set_timestamp(struct task_struct *task,
>> 	cgrp = perf_cgroup_from_task(task, ctx);
>> 	info = this_cpu_ptr(cgrp->info);
>> 	info->timestamp = ctx->timestamp;
>> +
>> +	/* set timestamp for ancestor cgroups */
>> +	if (cgrp->css.cgroup->level > 1) {
>> +		struct perf_cgroup_info *info = this_cpu_ptr(cgrp->info);
>> +		struct perf_event *event;
>> +
>> +		list_for_each_entry(event, &ctx->pinned_groups, group_entry)
>> +			perf_ancestor_cgroup_set_timestamp(event, cgrp, info);
>> +		list_for_each_entry(event, &ctx->flexible_groups, group_entry)
>> +			perf_ancestor_cgroup_set_timestamp(event, cgrp, info);
>> +	}
>> }
> 
> That doesn't make any kind of sense... if we're interested in ancestor
> groups, then WTH are you iterating _events_ ?
> 
> I'm expecting something like:
> 
> 	struct cgroup_subsys_state *css;
> 
> 	for (css = cgrp->css; css; css = css->parent) {
> 		/* fudge time */
> 	}

I guess this is what I have been looking for! Thanks Peter! 

V2 coming soon...

Song