linux-kernel - Re: [PATCH 1/2] kvm: use modern cpumask primitives, no cpumask

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Date:	Sun, 07 Dec 2008 17:52:36 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	kvm-devel <kvm@...r.kernel.org>, linux-kernel@...r.kernel.org,
	Mike Travis <travis@....com>, Avi Kivity <avi@...hat.com>
Subject: Re: [PATCH 1/2] kvm: use modern cpumask primitives, no cpumask_t
 on stack

Rusty Russell wrote:
> We're getting rid on on-stack cpumasks for large NR_CPUS.
>
> 1) Use cpumask_var_t and alloc_cpumask_var (a noop normally).  Fallback
>    code is inefficient but never happens in practice.
> 2) smp_call_function_mask -> smp_call_function_many
> 3) cpus_clear, cpus_empty, cpu_set -> cpumask_clear, cpumask_empty,
>    cpumask_set_cpu.
>
> --- linux-2.6.orig/virt/kvm/kvm_main.c
> +++ linux-2.6/virt/kvm/kvm_main.c
> @@ -358,11 +358,23 @@ static void ack_flush(void *_completed)
>  void kvm_flush_remote_tlbs(struct kvm *kvm)
>  {
>  	int i, cpu, me;
> -	cpumask_t cpus;
> +	cpumask_var_t cpus;
>  	struct kvm_vcpu *vcpu;
>  
>  	me = get_cpu();
> -	cpus_clear(cpus);
> +	if (!alloc_cpumask_var(&cpus, GFP_ATOMIC)) {
> +		/* Slow path on failure.  Call everyone. */
> +		for (i = 0; i < KVM_MAX_VCPUS; ++i) {
> +			vcpu = kvm->vcpus[i];
> +			if (vcpu)
> +				set_bit(KVM_REQ_TLB_FLUSH, &vcpu->requests);
> +		}
> +		++kvm->stat.remote_tlb_flush;
> +		smp_call_function_many(cpu_online_mask, ack_flush, NULL, 1);
> +		put_cpu();
> +		return;
> +	}
> +
>   

Wow, code duplication from Rusty. Things must be bad.

Since we're in a get_cpu() here, how about a per_cpu static cpumask 
instead? I don't mind the inefficient fallback, just the duplication.

-- 
error compiling committee.c: too many arguments to function

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