linux-kernel - Re: [patch 16/18] x86/apic: Convert 32bit to IPI shorthand static key

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Date:   Wed, 3 Jul 2019 22:34:33 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Nadav Amit <namit@...are.com>
cc:     LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>,
        Feng Tang <feng.tang@...el.com>
Subject: Re: [patch 16/18] x86/apic: Convert 32bit to IPI shorthand static
 key

Nadav,

On Wed, 3 Jul 2019, Nadav Amit wrote:
> > On Jul 3, 2019, at 3:54 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > void default_send_IPI_all(int vector)
> > {
> > -	if (apic_ipi_shorthand_off || vector == NMI_VECTOR) {
> > +	if (static_branch_likely(&apic_use_ipi_shorthand)) {
> > 		apic->send_IPI_mask(cpu_online_mask, vector);
> > 	} else {
> > 		__default_send_IPI_shortcut(APIC_DEST_ALLINC, vector);
> 
> It may be better to check the static-key in native_send_call_func_ipi() (and
> other callers if there are any), and remove all the other checks in
> default_send_IPI_all(), x2apic_send_IPI_mask_allbutself(), etc.

That makes sense. Should have thought about that myself, but hunting that
APIC emulation issue was affecting my brain obviously :)
 
>  void native_send_call_func_ipi(const struct cpumask *mask)
>  {
> -	cpumask_var_t allbutself;
> -
> -	if (!alloc_cpumask_var(&allbutself, GFP_ATOMIC)) {
> -		apic->send_IPI_mask(mask, CALL_FUNCTION_VECTOR);
> -		return;
> +	int cpu, this_cpu = smp_processor_id();
> +	bool allbutself = true;
> +	bool self = false;
> +
> +	for_each_cpu_and_not(cpu, cpu_online_mask, mask) {
> +
> +		if (cpu != this_cpu) {
> +			allbutself = false;
> +			break;
> +		}
> +		self = true;

That accumulates to a large iteration in the worst case. 

>  	}
>  
> -	cpumask_copy(allbutself, cpu_online_mask);
> -	__cpumask_clear_cpu(smp_processor_id(), allbutself);
> -
> -	if (cpumask_equal(mask, allbutself) &&
> +	if (allbutself && !self &&
>  	    cpumask_equal(cpu_online_mask, cpu_callout_mask))

Hmm. I overlooked that one. Need to take a deeper look.

>  		apic->send_IPI_allbutself(CALL_FUNCTION_VECTOR);
>  	else
>  		apic->send_IPI_mask(mask, CALL_FUNCTION_VECTOR);

Let me think about it for a while.

Thanks,

	tglx