Re: [PATCH 3/4] locking/osq_lock: From x_osq_lock/unlock to numa-aware lock/unlock.

[Waiman Long <longman@redhat.com>]

On 9/15/24 14:44, Uros Bizjak wrote:
>
>
> On 14. 09. 24 10:53, yongli-oc wrote:
>> According to the contention level, switches from x_osq_lock to
>> numa-aware osq_lock.
>> The numa-aware lock is a two level osq_lock.
>> The Makefile for dynamic numa-aware osq lock.
>>
>> Signed-off-by: yongli-oc <yongli-oc@zhaoxin.com>
>> ---
>>   kernel/locking/Makefile      |   1 +
>>   kernel/locking/numa.h        |  98 ++++++++
>>   kernel/locking/numa_osq.h    |  32 +++
>>   kernel/locking/x_osq_lock.c  | 332 +++++++++++++++++++++++++++
>>   kernel/locking/zx_numa_osq.c | 433 +++++++++++++++++++++++++++++++++++
>>   5 files changed, 896 insertions(+)
>>   create mode 100644 kernel/locking/numa.h
>>   create mode 100644 kernel/locking/numa_osq.h
>>   create mode 100644 kernel/locking/x_osq_lock.c
>>   create mode 100644 kernel/locking/zx_numa_osq.c
>
> ...
>
>> +    if (lock->numa_enable == OSQLOCKSTOPPING && old == 
>> OSQ_UNLOCKED_VAL)
>> +        old = OSQ_LOCKED_VAL;
>> +
>> +    for (;;) {
>> +        if (READ_ONCE(lock->tail16) == curr &&
>> +            cmpxchg(&lock->tail16, curr, old) == curr) {
>
> I would like to ask if there is any benefit to read the location two 
> times? cmpxchg() reads the location and skips the update when curr is 
> different from the value at the location by itself. Using 
> try_cmpxchg() can produce even more optimized code, since on x86 arch 
> CMPXCHG also sets ZF flag when operand 2 is equal to the value at 
> location (and update happens), and this flag can be used in a 
> conditional jump.

The major reason is for doing a read first before cmpxchg() is to avoid 
the overhead of an atomic operation in case the current task isn't the 
tail. We usually optimize for the case with a lot of incoming lockers 
where the chance of a match isn't particularly high.

Cheers,
Longman