Re: [PATCH v2] x86/hyper-v: micro-optimize send_ipi_one case

Roman Kagan
2019-10-25T14:05:50Z

On Fri, Oct 25, 2019 at 03:15:46PM +0200, Vitaly Kuznetsov wrote:
> When sending an IPI to a single CPU there is no need to deal with cpumasks.
> With 2 CPU guest on WS2019 I'm seeing a minor (like 3%, 8043 -> 7761 CPU
> cycles) improvement with smp_call_function_single() loop benchmark. The
> optimization, however, is tiny and straitforward. Also, send_ipi_one() is
> important for PV spinlock kick.
>
> I was also wondering if it would make sense to switch to using regular
> APIC IPI send for CPU > 64 case but no, it is twice as expesive (12650 CPU
> cycles for __send_ipi_mask_ex() call, 26000 for orig_apic.send_IPI(cpu,
> vector)).
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> Changes since v1:
> - Style changes [Roman, Joe]
> ---
> arch/x86/hyperv/hv_apic.c | 13 ++++++++++---
> arch/x86/include/asm/trace/hyperv.h | 15 +++++++++++++++
> 2 files changed, 25 insertions(+), 3 deletions(-)

Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>