Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios in PLE handler

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 09/24/2012 05:04 PM, Peter Zijlstra wrote:
> On Fri, 2012-09-21 at 17:29 +0530, Raghavendra K T wrote:
>> In some special scenarios like #vcpu<= #pcpu, PLE handler may
>> prove very costly, because there is no need to iterate over vcpus
>> and do unsuccessful yield_to burning CPU.
>
> What's the costly thing? The vm-exit, the yield (which should be a nop
> if its the only task there) or something else entirely?
>
Both vmexit and yield_to() actually,

because unsuccessful yield_to() overall is costly in PLE handler.

This is because when we have large guests, say 32/16 vcpus, and one
vcpu is holding lock, rest of the vcpus waiting for the lock, when they
do PL-exit, each of the vcpu try to iterate over rest of vcpu list in
the VM and try to do directed yield (unsuccessful). (O(n^2) tries).

this results is fairly high amount of cpu burning and double run queue
lock contention.

(if they were spinning probably lock progress would have been faster).
As Avi/Chegu Vinod had felt it is better to avoid vmexit itself, which
seems little complex to achieve currently.