Re: [patch V2 21/25] x86/smp: Enhance native_send_call_func_ipi()

[Nadav Amit]

> On Jul 4, 2019, at 8:52 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
> Nadav noticed that the cpumask allocations in native_send_call_func_ipi()
> are noticeable in microbenchmarks.
> 
> Use the new cpumask_or_equal() function to simplify the decision whether
> the supplied target CPU mask is either equal to cpu_online_mask or equal to
> cpu_online_mask except for the CPU on which the function is invoked.
> 
> cpumask_or_equal() or's the target mask and the cpumask of the current CPU
> together and compares it to cpu_online_mask.
> 
> If the result is false, use the mask based IPI function, otherwise check
> whether the current CPU is set in the target mask and invoke either the
> send_IPI_all() or the send_IPI_allbutselt() APIC callback.
> 
> Make the shorthand decision also depend on the static key which enables
> shorthand mode. That allows to remove the extra cpumask comparison with
> cpu_callout_mask.
> 
> Reported-by: Nadav Amit <namit@xxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
> V2: New patch
> ---
> arch/x86/kernel/apic/ipi.c |   24 +++++++++++-------------
> 1 file changed, 11 insertions(+), 13 deletions(-)
> 
> --- a/arch/x86/kernel/apic/ipi.c
> +++ b/arch/x86/kernel/apic/ipi.c
> @@ -83,23 +83,21 @@ void native_send_call_func_single_ipi(in
> 
> void native_send_call_func_ipi(const struct cpumask *mask)
> {
> -	cpumask_var_t allbutself;
> +	if (static_branch_likely(&apic_use_ipi_shorthand)) {
> +		unsigned int cpu = smp_processor_id();
> 
> -	if (!alloc_cpumask_var(&allbutself, GFP_ATOMIC)) {
> -		apic->send_IPI_mask(mask, CALL_FUNCTION_VECTOR);
> +		if (!cpumask_or_equal(mask, cpumask_of(cpu), cpu_online_mask))
> +			goto sendmask;
> +
> +		if (cpumask_test_cpu(cpu, mask))
> +			apic->send_IPI_all(CALL_FUNCTION_VECTOR);
> +		else if (num_online_cpus() > 1)
> +			apic->send_IPI_allbutself(CALL_FUNCTION_VECTOR);
> 		return;
> 	}
> 
> -	cpumask_copy(allbutself, cpu_online_mask);
> -	cpumask_clear_cpu(smp_processor_id(), allbutself);
> -
> -	if (cpumask_equal(mask, allbutself) &&
> -	    cpumask_equal(cpu_online_mask, cpu_callout_mask))
> -		apic->send_IPI_allbutself(CALL_FUNCTION_VECTOR);
> -	else
> -		apic->send_IPI_mask(mask, CALL_FUNCTION_VECTOR);
> -
> -	free_cpumask_var(allbutself);
> +sendmask:
> +	apic->send_IPI_mask(mask, CALL_FUNCTION_VECTOR);
> }
> 
> #endif /* CONFIG_SMP */

It does look better and simpler than my solution.