Re: [PATCH v4 0/3] locking/rwsem: Rwsem rearchitecture part 0

[Waiman Long]

On 02/14/2019 05:37 AM, Peter Zijlstra wrote:
> On Wed, Feb 13, 2019 at 05:00:14PM -0500, Waiman Long wrote:
>> v4:
>>  - Remove rwsem-spinlock.c and make all archs use rwsem-xadd.c.
>>
>> v3:
>>  - Optimize __down_read_trylock() for the uncontended case as suggested
>>    by Linus.
>>
>> v2:
>>  - Add patch 2 to optimize __down_read_trylock() as suggested by PeterZ.
>>  - Update performance test data in patch 1.
>>
>> The goal of this patchset is to remove the architecture specific files
>> for rwsem-xadd to make it easer to add enhancements in the later rwsem
>> patches. It also removes the legacy rwsem-spinlock.c file and make all
>> the architectures use one single implementation of rwsem - rwsem-xadd.c.
>>
>> Waiman Long (3):
>>   locking/rwsem: Remove arch specific rwsem files
>>   locking/rwsem: Remove rwsem-spinlock.c & use rwsem-xadd.c for all
>>     archs
>>   locking/rwsem: Optimize down_read_trylock()
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>
> with the caveat that I'm happy to exchange patch 3 back to my earlier
> suggestion in case Will expesses concerns wrt the ARM64 performance of
> Linus' suggestion.

I inserted a few lock event counters into the rwsem trylock code:

static inline int __down_read_trylock(struct rw_semaphore *sem)
{
ÂÂÂÂÂÂÂ /*
ÂÂÂÂÂÂÂÂ * Optimize for the case when the rwsem is not locked at all.
ÂÂÂÂÂÂÂÂ */
ÂÂÂÂÂÂÂ long tmp = RWSEM_UNLOCKED_VALUE;

ÂÂÂÂÂÂÂ lockevent_inc(rwsem_rtrylock);
ÂÂÂÂÂÂÂ do {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ tmp + RWSEM_ACTIVE_READ_BIAS)) {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ rwsem_set_reader_owned(sem);
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return 1;
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ }
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ lockevent_inc(rwsem_rtrylock_retry);
ÂÂÂÂÂÂÂ } while (tmp >= 0);
ÂÂÂÂÂÂÂ lockevent_inc(rwsem_rtrylock_fail);
ÂÂÂÂÂÂÂ return 0;
}

static inline int __down_write_trylock(struct rw_semaphore *sem)
{
ÂÂÂÂÂÂÂ long tmp;

ÂÂÂÂÂÂÂ lockevent_inc(rwsem_wtrylock);
ÂÂÂÂÂÂÂ tmp = atomic_long_cmpxchg_acquire(&sem->count, RWSEM_UNLOCKED_VALUE,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ RWSEM_ACTIVE_WRITE_BIAS);
ÂÂÂÂÂÂÂ if (tmp == RWSEM_UNLOCKED_VALUE) {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ rwsem_set_owner(sem);
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return true;
ÂÂÂÂÂÂÂ }
ÂÂÂÂÂÂÂ lockevent_inc(rwsem_wtrylock_fail);
ÂÂÂÂÂÂÂ return false;
}

I booted the new kernel on a 4-socket 56-core 112-thread Broadwell
system. The counter values

1) After bootup:

rwsem_rtrylock=784029
rwsem_rtrylock_fail=59
rwsem_rtrylock_retry=394
rwsem_wtrylock=18284
rwsem_wtrylock_fail=230

2) After parallel kernel build (-j112):

rwsem_rtrylock=338667559
rwsem_rtrylock_fail=18
rwsem_rtrylock_retry=51
rwsem_wtrylock=17016332
rwsem_wtrylock_fail=98058

At least for these two use cases, try-for-ownership as suggested by
Linus is the right choice.

Cheers,
Longman