Re: [PATCH 1/2] kvm: use modern cpumask primitives, no cpumask_t on stack

[Mike Travis <travis@sgi.com>]

Avi Kivity wrote:
> 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.
> 

One thing to note is that when CPUMASK_OFFSTACK=n, then alloc_cpumask_var
returns a constant 1 and the duplicate code is not even compiled.