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

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Date:	Wed, 8 Jul 2015 13:42:50 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Cong Wang <cwang@...pensource.com>
Subject: Re: [PATCH v2] sched: introduce sched_switch_post trace event

On Mon, Jul 6, 2015 at 5:15 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Mon,  6 Jul 2015 12:15:45 -0700
> Cong Wang <xiyou.wangcong@...il.com> 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`).
>
> I just want to understand this correctly. Does perf sched only listen
> to events that are executed by the task in a particular cgroup? There's
> no way to say "check sched_switch field next"?
>

perf_event cgroup needs to be specified in cmdline and `perf sched`
doesn't support that currently, I wrote my own tool to do this.
(I have some patch to add it to `perf sched`)

As I replied in the previous thread, we can certainly check if a process
belongs to a cgroup by tracking the PID's, but that is not easy.

>
> This looks identical to trace_sched_switch. Please convert both to a
> DECLARE_EVENT_CLASS() and DEFINE_EVENT()s.
>

Not identical, I rename 'next' to 'curr' since switch is done 'next'
becomes meaningless.

Thanks.
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