Re: [PATCH] xen: correctly check for pending events when restoring irq flags

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Thu, Apr 26, 2012 at 07:44:06PM +0100, David Vrabel wrote:
> From: David Vrabel <david.vrabel@citrix.com>
> 
> In xen_restore_fl_direct(), xen_force_evtchn_callback() was being
> called even if no events were pending.  This resulted in (depending on
> workload) about a 100 times as many xen_version hypercalls as
> necessary.
> 
> Fix this by correcting the sense of the conditional jump.
> 
> This seems to give a significant performance benefit for some
> workloads.
> 

Will put stable@kernel.org here as well.

Thx!
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> ---
> I'd prefer to do more testing and benchmarking but the gains appear to
> be significant.  e.g., lmbench3's 'lat_proc fork' test has improved
> from 404 us to 357 us (~11% faster).
> ---
>  arch/x86/xen/xen-asm.S |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/xen/xen-asm.S b/arch/x86/xen/xen-asm.S
> index 79d7362..3e45aa0 100644
> --- a/arch/x86/xen/xen-asm.S
> +++ b/arch/x86/xen/xen-asm.S
> @@ -96,7 +96,7 @@ ENTRY(xen_restore_fl_direct)
>  
>  	/* check for unmasked and pending */
>  	cmpw $0x0001, PER_CPU_VAR(xen_vcpu_info) + XEN_vcpu_info_pending
> -	jz 1f
> +	jnz 1f
>  2:	call check_events
>  1:
>  ENDPATCH(xen_restore_fl_direct)
> -- 
> 1.7.2.5