Re: [PATCH] sched: split sched_switch trace event into two

[Cong Wang]

On Thu, Jun 25, 2015 at 12:48 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Wed, Jun 24, 2015 at 04:19:33PM -0700, Cong Wang wrote:
>> Currently we only have one sched_switch trace event
>> for task switching, which is generated very early during
>> task switch. When we try to monitor per-container events,
>> this is not what we expect.
>
> Adjust your expectations?
>
>> For example, we have a process A which is in the cgroup
>> we monitor, and process B which isn't, when kernel switches
>> from B to A, the sched_switch event is not recorded for this
>> cgroup since it belongs to B (current process is still B
>> util we finish the switch), but we require this event to
>> signal that process A in this cgroup gets scheduled. This is
>> crucial for calculating schedule latency.
>
> I don't get it. This is global data in the root pid-space.
>
> The switch data includes both the previous and the next task. Just look
> up their corresponding cgroups and be done with it.


This is exactly how I _workaround_ this issue for now. :)

But it is not that easy, because we need to track new processes
and dying processes to maintain the pid list.

>
> If you cannot get what you want from it, you're doing it wrong.

The problem is why user-space has to do that (as mentioned
above) just to track their cgroup since we already have perf_event
cgroup.

All what I want is tracking several sched events within a given
perf_event cgroup and calculating the sched latency (like how
`perf sched` does), and sched_switch is an exception since
it is related with two processes which can be in two different
perf_event cgroups, and since it is generated early during
task switch, all sched_switch events belong to 'prev' task, we
lose the track of the sched_switch for 'next' task.

Fixing it in kernel is easier than workaround in user-space,
this is why I come up with this patch.

Thanks.
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