Messages in this thread | | | Subject | Re: [RFC] Fast assurate clock readable from user space and NMI handler | From | Daniel Walker <> | Date | Sat, 24 Feb 2007 10:06:09 -0800 |
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On Sat, 2007-02-24 at 11:19 -0500, Mathieu Desnoyers wrote: > Hi, > > I am trying to improve the Linux kernel time source so it can be read > without seqlock from NMI handlers. I have also seen some interest for > such an accurate monotonic clock readable from user space. It mainly > implies an atomic update of the time value. I am also trying to figure a > way to support architectures with multiple CPUs with non-synchronized > TSCs. > > I would like to have your comments on the following idea. > > Thanks in advance, > > Mathieu > > > Monotonic accurate time > > The goal of this design is to provide a monotonic time : > > Readable from userspace without a system call > Readable from NMI handler > Readable without disabling interrupts > Readable without disabling preemption > Only one clock source (most precise available : tsc) > Support architectures with variable TSC frequency.
I don't think you could use only the tsc. From reviewing John, and Thomas work it's pretty clear the TSC isn't going to work correctly all the time.
> /* On frequency change event */ > /* In irq context */ > void freq_change_cb(unsigned int new_freq) > {
It's possible for the TSC to change frequencies without notification. It can also completely stop when the system goes idle.
> /* Userspace */ > /* Export all this data to user space through the vsyscall page. Use a function > * like read_time to read the walltime. This function can be implemented as-is > * because it doesn't need to disable preemption. */
What would be the benefit of using this over the vsyscall gettimeofday() from userspace ?
Daniel
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