Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

[Avi Kivity <avi@redhat.com>]

On 07/11/2012 01:17 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

We could make it available via shared memory, updated using preempt
notifiers.  Of course piling on more pv makes this less attractive.

> - 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,

What name would you suggest?

> yes the data structure itself seems based on the algorithm
> and not on arch specific things. That should work. 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.

ARM doesn't have an instruction for cpu_relax(), so it can't intercept
it.  Given ppc's dislike of overcommit, and the way it implements
cpu_relax() by adjusting hw thread priority, I'm guessing it doesn't
intercept those either, but I'm copying the ppc people in case I'm
wrong.  So it's s390 and x86.

> So this would probably improve guests that uses cpu_relax, for example
> stop_machine_run. I have no measurements, though.

smp_call_function() too (though that can be converted to directed yield
too).  It seems worthwhile.

-- 
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