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Message-ID: <1f1b08da0809221330r11f3b82l48509dbc1ea4bb63@mail.gmail.com>
Date: Mon, 22 Sep 2008 13:30:33 -0700
From: "john stultz" <johnstul@us.ibm.com>
To: "George Nychis" <gnychis@cmu.edu>
Subject: Re: printing current system time from kernel space
Cc: linux-kernel@vger.kernel.org
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On Mon, Sep 22, 2008 at 1:28 PM, john stultz <johnstul@us.ibm.com> wrote:
> On Sun, Sep 21, 2008 at 10:18 PM, George Nychis <gnychis@cmu.edu> wrote:
>> I am looking to measure the latency of USB data between kernel space and
>> user space.  The user space driver uses a URB to get data from the device to
>> the kernel and finally to user space.
>>
>> To measure this latency, I was thinking of printing the current system time
>> when a read occurs/succeeds in drivers/usb/core/devio.c at the function
>> usbdev_read(), and then again in user space when the URB succeeds in
>> reading.  Then, I could subtract the two times to get the latency.
>>
>> I spent some time googling, but could not find out how or if it is possible
>> to read the current system time in kernel space.  I could insert a printk()
>> somewhere in usbdev_read() then.
>>
>> If it is not possible to read the current system time, is there some other
>> shared clock between kernel and user space that I could use for this?
>
> Kernel: getnstimeofday()
> Userland: clock_gettime(CLOCK_MONOTONIC, ...)
>
> Those two should give you the same data. So printing timespecs from
> kernel space that come from getnstimeofday() and comparing it to
> userland clock_gettime(CLOCK_MONOTONIC,...) hopefully will give you
> what you want.

Gah! Typed too fast. The above is wrong. You want to use
clock_gettime(CLOCK_REALTIME,...)  not CLOCK_MONOTONIC with
getnstimeofday().

Sorry for the confusion.
-john