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

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Daniel Walker (dwalker@mvista.com) wrote:
> 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.
> 

Ok, if there are other high precision timers we can use, I guess it may
be better to fall back on them.

> > /* 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.
> 

Hrm, I see. I though those freq change without notification would happen
rarely and could be dealt by resynchronizing the CPUs. I guess I was
wrong.

> > /* 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 ? 
> 

So we can get a monotonic, non NTP corrected timestamp quickly from user
space without going through a system call. Are there other alternatives ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Candidate, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68