Re: [PATCH 2/6] KVM: use kvm_{test,clear}_request instead of {test,clear}_bit

[Radim KrÄmÃÅ]

2017-04-07 14:24+0200, Radim KrÄmÃÅ:
> 2017-04-07 12:55+0200, Christian Borntraeger:
>> On 04/06/2017 10:20 PM, Radim KrÄmÃÅ wrote:
>>>  static inline bool kvm_check_request(int req, struct kvm_vcpu *vcpu)
>>>  {
>>> -	if (test_bit(req, &vcpu->requests)) {
>>> -		clear_bit(req, &vcpu->requests);
>>> +	if (kvm_test_request(req, vcpu)) {
>>> +		kvm_clear_request(req, vcpu);
>> 
>> This looks fine. I am just asking myself why we do not use
>> test_and_clear_bit? Do we expect gcc to merge all test bits as
>> a fast path? This does not seem to work as far as I can tell and
>> almost everybody does a fast path like in
> 
> test_and_clear_bit() is a slower operation even if the test is false (at
> least on x86), because it needs to be fully atomic.
> 
>> arch/s390/kvm/kvm-s390.c:
>>         if (!vcpu->requests)
>>                 return 0;
>> 
>> arch/x86/kvm/x86.c:
>>     if (vcpu->requests) {
> 
> We'll mostly have only one request set, so splitting the test_and_clear
> improves the performance of many subsequent tests_and_clear()s even if
> the compiler doesn't optimize.
> 
> GCC couldn't even optimize if we used test_and_clear_bit(), because that
> instruction adds barriers, but the forward check for vcpu->requests is
> there because we do not trust the optimizer to do it for us and it would
> make a big difference.

Ugh, I started thinking that bitops are not atomic because I looked at
wrong boot/bitops.h by mistake.  The compiler cannot merge test_bit()s,
but the speed difference holds.