linux-kernel - Re: printing current system time from kernel space

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Date:	Mon, 22 Sep 2008 13:28:17 -0700
From:	"john stultz" <johnstul@...ibm.com>
To:	"George Nychis" <gnychis@....edu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: printing current system time from kernel space

On Sun, Sep 21, 2008 at 10:18 PM, George Nychis <gnychis@....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.

Let me know if I misunderstood.

thanks
-john
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