Messages in this thread | | | Date | Mon, 22 Sep 2008 16:33:43 -0400 | From | George Nychis <> | Subject | Re: printing current system time from kernel space |
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john stultz wrote: > 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 >
Thanks John!
This is exactly what I am looking for.
- George
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