On 07/11/2012 03:47 PM, Christian Borntraeger wrote:
On 11/07/12 11:06, Avi Kivity wrote:
[...]
Almost all s390 kernels use diag9c (directed yield to a given guest cpu) for
spinlocks, though.
Perhaps x86 should copy this.
See arch/s390/lib/spinlock.c
The basic idea is using several heuristics:
- loop for a given amount of loops
- check if the lock holder is currently scheduled by the hypervisor
(smp_vcpu_scheduled, which uses the sigp sense running instruction)
Dont know if such thing is available for x86. It must be a lot cheaper
than a guest exit to be useful
Unfortunately we do not have information on lock-holder.
- if lock holder is not running and we looped for a while do a directed
yield to that cpu.
So there is no win here, but there are other cases were diag44 is used, e.g.
cpu_relax.
I have to double check with others, if these cases are critical, but for now,
it seems
that your dummy implementation for s390 is just fine. After all it is a no-op
until
we implement something.
Does the data structure make sense for you? If so we can move it to
common code (and manage it in kvm_vcpu_on_spin()). We can guard it with
CONFIG_KVM_HAVE_CPU_RELAX_INTERCEPT or something, so other archs don't
have to pay anything.
Ignoring the name, yes the data structure itself seems based on the algorithm
and not on arch specific things. That should work.
Ok. can you please elaborate, on the flow.
If we move that to common
code then s390 will use that scheme automatically for the cases were we call
kvm_vcpu_on_spin(). All others archs as well.
So this would probably improve guests that uses cpu_relax, for example
stop_machine_run. I have no measurements, though.
Christian
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
