Re: [RFC] Fast assurate clock readable from user space and NMI handler

[Daniel Walker <dwalker@mvista.com>]

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