ACK: [PATCH][SRU][Xenial] KVM: x86: lower default for halt_poll_ns

[Colin Ian King]

On 26/01/18 16:41, Victor Tapia wrote:
> From: Paolo Bonzini <pbonzini at redhat.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1724614
> 
> In some fio benchmarks, halt_poll_ns=400000 caused CPU utilization to
> increase heavily even in cases where the performance improvement was
> small.  In particular, bandwidth divided by CPU usage was as much as
> 60% lower.
> 
> To some extent this is the expected effect of the patch, and the
> additional CPU utilization is only visible when running the
> benchmarks.  However, halving the threshold also halves the extra
> CPU utilization (from +30-130% to +20-70%) and has no negative
> effect on performance.
> 
> Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
> Signed-off-by: Radim Krčmář <rkrcmar at redhat.com>
> (backported from commit b401ee0b85a53e89739ff68a5b1a0667d664afc9)
> Signed-off-by: Victor Tapia <victor.tapia at canonical.com>
> ---
>  arch/x86/include/asm/kvm_host.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 40be363..4028beb 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -42,7 +42,7 @@
>  
>  #define KVM_PIO_PAGE_OFFSET 1
>  #define KVM_COALESCED_MMIO_PAGE_OFFSET 2
> -#define KVM_HALT_POLL_NS_DEFAULT 400000
> +#define KVM_HALT_POLL_NS_DEFAULT 200000
>  
>  #define KVM_IRQCHIP_NUM_PINS  KVM_IOAPIC_NUM_PINS
>  
> 
Can't argue with the documented performance increase, so it seems
safe enough a change to me.

Acked-by: Colin Ian King <colin.king at canonical.com>