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

[Roman Kagan <rkagan@virtuozzo.com>]

On Thu, Oct 24, 2019 at 05:21:52PM +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)).

Is it with APICv or emulated apic?

> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/x86/hyperv/hv_apic.c           | 22 +++++++++++++++++++---
>  arch/x86/include/asm/trace/hyperv.h | 15 +++++++++++++++
>  2 files changed, 34 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/hyperv/hv_apic.c b/arch/x86/hyperv/hv_apic.c
> index e01078e93dd3..847f9d0328fe 100644
> --- a/arch/x86/hyperv/hv_apic.c
> +++ b/arch/x86/hyperv/hv_apic.c
> @@ -194,10 +194,26 @@ static bool __send_ipi_mask(const struct cpumask *mask, int vector)
>  
>  static bool __send_ipi_one(int cpu, int vector)
>  {
> -	struct cpumask mask = CPU_MASK_NONE;
> +	int ret;
>  
> -	cpumask_set_cpu(cpu, &mask);
> -	return __send_ipi_mask(&mask, vector);
> +	trace_hyperv_send_ipi_one(cpu, vector);
> +
> +	if (unlikely(!hv_hypercall_pg))
> +		return false;
> +
> +	if (unlikely((vector < HV_IPI_LOW_VECTOR) ||
> +		     (vector > HV_IPI_HIGH_VECTOR)))
> +		return false;

I guess 'ulikely' is unnecessary in these cases.

> +
> +	if (cpu >= 64)
> +		goto do_ex_hypercall;
> +
> +	ret = hv_do_fast_hypercall16(HVCALL_SEND_IPI, vector,
> +				     BIT_ULL(hv_cpu_number_to_vp_number(cpu)));
> +	return ((ret == 0) ? true : false);

D'oh.  Isn't "return ret == 0;" or just "return ret;" good enough?

These tiny nitpicks are no reason to hold the patch though, so

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