On Mon, 2009-05-11 at 17:24 +0300, Avi Kivity wrote: > > I.e. this is a somewhat poor solution as far as scheduling goes. But > > i'm wondering what the CPU side does. Can REP-NOP really take > > thousands of cycles? If yes, under what circumstances? > > > > The guest is running rep-nop in a loop while trying to acquire a > spinlock. The hardware detects this (most likely, repeated rep-nop with > the same rip) and exits. We can program the loop count; obviously if > we're spinning for only a short while it's better to keep spinning while > hoping the lock will be released soon. > > The idea is to detect that the guest is not making forward progress and > yield. If I could tell the scheduler, you may charge me a couple of > milliseconds, I promise not to sue, that would be ideal. Other tasks > can become eligible, hopefully the task holding the spinlock, and by the > time we're scheduled back the long running task will have finished and > released the lock. > > For newer Linux as a guest we're better off paravirtualizing this, so we > can tell the host which vcpu holds the lock; in this case kvm will want > to say, take a couple milliseconds off my account and transfer it to > this task (so called directed yield). However there's no reason to > paravirtualize all cpu_relax() calls. So we're now officially giving up on (soft) realtime virtualization? -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
