printing current system time from kernel space

[George Nychis <gnychis@cmu.edu>]

Hi all,

Please CC me on any responses.

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?

Thank you!
George