Re: [Qemu-devel] [PATCH v5 07/18] qemu-thread: add simple test-and-set spinlock

[Sergey Fedorov]

On 18/05/16 23:52, Emilio G. Cota wrote:
> On Wed, May 18, 2016 at 22:51:09 +0300, Sergey Fedorov wrote:
>> On 14/05/16 06:34, Emilio G. Cota wrote:
>>> +static inline void qemu_spin_lock(QemuSpin *spin)
>>> +{
>>> +    while (atomic_test_and_set_acquire(&spin->value)) {
>> A possible optimization might be using unlikely() here, copmare:
> Testing with a spinlock-heavy workload reveals a little improvement:
>
> taskset -c 0 tests/qht-bench \
>       -d 5 -n 1 -u 100 -k 4096 -K 4096 -l 4096 -r 4096 -s 4096
>
> I'm running this 10 times. Results in Mops/s:
> Head                  31.283 +- 0.190557661148069
> while (unlikely)      31.397 +- 0.107501937967028
> if (likely) + while   31.524 +- 0.219605707272527 
>
> The last case does:
>     if (likely(__sync_lock_test_and_set(&spin->value, true) == false)) {
>         return;
>     }
>     while (__sync_lock_test_and_set(&spin->value, true)) {
>         while (atomic_read(&spin->value)) {
>             cpu_relax();
>         }
>     }
>
> Although I don't like how this will do the TAS twice if the lock is
> contended.
>
> I'll just add the unlikely() to while().

Great!

>
>>> +static inline int qemu_spin_trylock(QemuSpin *spin)
>>> +{
>>> +    if (atomic_test_and_set_acquire(&spin->value)) {
>>> +        return -EBUSY;
>>> +    }
>>> +    return 0;
>>> +}
>> Here we could also benefit from unlikely(), I think.
> I never liked this branch in _trylock, because there will
> be a branch anyway around the function. How about:
>
> static inline bool qemu_spin_trylock(QemuSpin *spin)
> {
>     return __sync_lock_test_and_set(&spin->value, true);
> }
>
> We don't return EBUSY, which nobody cares about anyway; callers
> will still do if (!trylock). With this we save a branch,
> and let callers sprinkle likely/unlikely based on how contented
> they expect the lock to be.

It's "static inline" anyway, so that shouldn't matter much if we have
that "if". But you're right, let's save likely/unlikely for the user of
the function.

Kind regards,
Sergey