[RFC -v3 PATCH 0/3] directed yield for Pause Loop Exiting

[Rik van Riel]

When running SMP virtual machines, it is possible for one VCPU to be
spinning on a spinlock, while the VCPU that holds the spinlock is not
currently running, because the host scheduler preempted it to run
something else.

Both Intel and AMD CPUs have a feature that detects when a virtual
CPU is spinning on a lock and will trap to the host.

The current KVM code sleeps for a bit whenever that happens, which
results in eg. a 64 VCPU Windows guest taking forever and a bit to
boot up.  This is because the VCPU holding the lock is actually
running and not sleeping, so the pause is counter-productive.

In other workloads a pause can also be counter-productive, with
spinlock detection resulting in one guest giving up its CPU time
to the others.  Instead of spinning, it ends up simply not running
much at all.

This patch series aims to fix that, by having a VCPU that spins
give the remainder of its timeslice to another VCPU in the same
guest before yielding the CPU - one that is runnable but got 
preempted, hopefully the lock holder.

v3:
- more cleanups
- change to Mike Galbraith's yield_to implementation
- yield to spinning VCPUs, this seems to work better in some
  situations and has little downside potential
v2:
- make lots of cleanups and improvements suggested
- do not implement timeslice scheduling or fairness stuff
  yet, since it is not entirely clear how to do that right
  (suggestions welcome)
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