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SubjectRe: An small ftrace enhancement idea
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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

在 2013年10月31日,下午1:50,Steven Rostedt <rostedt@goodmis.org> 写道:

> 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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