RE: [Xen-devel] [PATCH] Yield to VCPU hcall, spinlock yielding

[Orran Y Krieger <okrieg@xxxxxxxxxx>]

xen-devel-bounces@xxxxxxxxxxxxxxxxxxx wrote on 06/07/2005
06:38:14 AM:

> Orran, 
> 
> Are you assuming the pre-emption notification is going to get propagated
> to user-space as a signal, and that user space applications would
be
> modified to take advantage of the signal? (possibly this could be
hidden
> in the pthread library?} Since the kernel doesn't know when user space
> has an application lock or not, that's going to be a lot of signals.

Nope, we reviewed a couple of alternatives for notification
to user level and for now are not persuing any of them.  The pre-emption
notification goes to the kernel, and its an OS specific thing what it chooses
to do with it. Our current plan in Linux is to run a high priority thread
in the kernel which makes a hypervisor call to yield back the processor.
Hence, the application thread context is not hidden in the hypervisor,
but the application thread is marked by linux as ready to run.  Now
if the thread is a high priority thread holding an application lock normal
linux mechanisms will cause it to be scheduled on some other processor.
 Without the preemption notification, or if we yield back at interrupt
level, the application thread register state is buried in the hypervisor,
and of course the thread appears to Linux to be running...

Everything seems to just fall out nicely in Linux
as long as we make the call back on a kernel thread. Different OSes, of
course, will have different techniques they can use given a notification
of an attempt to preempt. 

    -- Orran
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