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    SubjectRe: printing current system time from kernel space
    On Sun, Sep 21, 2008 at 10:18 PM, George Nychis <> 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.


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