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

[Waiman Long]

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@xxxxxxxxxxx>
> > ---
> >   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