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List:       linux-kernel
Subject:    Re: An small ftrace enhancement idea
From:       Kenneth Lee <nek.in.cn () gmail ! com>
Date:       2013-10-31 23:40:12
Message-ID: 1B57F15E-49C6-4D62-8A2F-A3D54F783593 () gmail ! com
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Thank you, Steve, 

Yes, with a separated instance, I can measure the latency for a stimulation while capture the other \
schedule events which I am interesting in. This is a better solution. I don¡¯t know this ¡°instance¡± \
stuff before. I don¡¯t need to create another axe. I am sorry for my ignorance. 

Thanks and regards.

--Kenneth

ÔÚ 2013Äê10Ô 31ÈÕ£¬Ï Îç1:50£¬Steven Rostedt <rostedt@goodmis.org> дµÀ£º

> On Wed, 30 Oct 2013 15:39:50 -0700
> Kenneth Lee <nek.in.cn@gmail.com> wrote:
> 
> > Dear Steven,
> > 
> > I want to add a new function to ftrace subsystem. Sometimes, we will face such a problem: system do \
> > not response to the input on time one to two times everyday. It is not easy to capture because it \
> > rarely happens. So I want to add a function to the kernel. If I have such problem, I insert a kernel \
> > module, who add a hook to the position that receive the input and another to the position that \
> > response to the input (with a session id if necessary). And I can compare the time between them and \
> > if the period is longer then a pre-set threshold, I can give a signal to a user helper application \
> > (maybe a script waiting on the file), which then can save the trace event to a file for later \
> > inspection.
> 
> I'm a little confused in what you want.
> 
> 
> > 
> > 
> > 
> > The user helper script may look like this:
> > 
> > 
> > 
> > #/bin/sh
> > 
> > 
> > 
> > echo ¡®sched:*¡¯ > /sys/kernel/debug/tracing/set_event
> > 
> > modprobe delay_inspector.ko threshold=500
> > 
> > cat /sys/kernel/debug/tracing/waiter #wait for signal
> > 
> > cp /sys/kernel/debug/tracing/trace /var/log/delay_infomation
> > 
> > 
> > 
> > 
> > 
> > It looks like a standalone function. But I don¡¯t have place to put it. Do you think I can implement \
> > it in ftrace? And do you think if there are better solution? 
> 
> You want something to wake up if it takes too long before an event
> happens?
> 
> If so, why not just use a select() on the trace_pipe and if it times
> out, then dump the trace. You can even set up a separate instance.
> 
> (this is waiting for a schedule switch to pid 1)
> 
> cd /sys/kernel/debug/tracing
> mkdir instances/mine
> echo 'next_pid == 1' > instances/mine/events/sched/sched_switch/filter
> echo 1 > instances/mine/events/sched/sched_switch/enable
> 
> 
> The in a userspace program, I open "instances/mine/events/trace_pipe"
> and run a select() on that file descriptor with a given timeout. If the
> event does not happen within the expected time frame, the select
> returns zero, and this userspace program can deal with it.
> 
> Is that the functionality you are trying to achieve?
> 
> -- Steve

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