linux-kernel - Re: [PATCH v2] sched: introduce sched_switch

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Date:	Wed, 8 Jul 2015 13:48:33 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Cong Wang <cwang@...pensource.com>
Subject: Re: [PATCH v2] sched: introduce sched_switch_post trace event

On Mon, Jul 6, 2015 at 11:45 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Jul 06, 2015 at 12:15:45PM -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 perf
>> events, this is not what we expect.
>>
>> 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 (like `perf sched`).
>>
>> Ideally, we need to split the sched_switch event into two:
>> sched_in event before we perform the switch, and sched_out
>> event after we perform the switch. However, for compatibility,
>> we can not change the sched_switch event. So before we have
>> trace event alias, we can just reuse sched_switch and introduce
>> sched_switch_post event instead.
>
> No.. its still horrible.
>
> You're trying to solve perf problems with ftrace; this cannot work.

It works for tools like `perf sched` which only listens to trace events. :)

>
> Does this patch by Adrian work for you? I think it solves this problem
> and a bunch of others.
>
> lkml.kernel.org/r/1435927962-32417-2-git-send-email-adrian.hunter@...el.com

Ah, probably, as long as we have some event after sched switch,
no matter it's perf event trace event, it should work for me too.

The downside of using a perf event is that it is _a bit_ harder to parse
a new perf event than reusing the current code to parse a new trace
event.

On the other hand, only perf events have cgroup, trace events
AFAIK don't have cgroup. So in this aspect, it is right to generate
a perf event after switch.

Thanks.
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