Re: printing current system time from kernel space

[George Nychis]

john stultz wrote:
> On Mon, Sep 22, 2008 at 1:28 PM, john stultz <johnstul@xxxxxxxxxx> wrote:
> > On Sun, Sep 21, 2008 at 10:18 PM, George Nychis <gnychis@xxxxxxx> 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

Thanks John!

This is exactly what I am looking for.

- George
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