Re: [PATCH v4 next 0/9] locking/osq_lock: Optimisations to osq_lock code

[David Laight <david.laight.linux@gmail.com>]

On Thu, 10 Sep 2026 09:45:47 +0000
Haakon Bugge <haakon.bugge@oracle.com> wrote:

> > On 9 Sep 2026, at 22:33, Waiman Long <longman@redhat.com> wrote:  
> 
> [snip]
> 
> > > Could you make that change to the existing code and rerun the test 
> > > again on arm64 to see if it can pass?   
> > 
> > osq_lock/unlock() is special in the sense that lock transfer can happen 
> > either in the lock cacheline or the node->locked cacheline. Try the 
> > patch below to see if it helps to pass the test.
> > 
> > Thanks,
> > Longman
> > 
> > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> > index b4233dc2c2b0..51cecf297692 100644
> > --- a/kernel/locking/osq_lock.c
> > +++ b/kernel/locking/osq_lock.c
> > @@ -143,7 +143,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> >          * is implemented with a monitor-wait. vcpu_is_preempted() 
> > relies on
> >          * polling, be careful.
> >          */
> > -       if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
> > +       if (smp_cond_load_acquire(&node->locked, VAL || need_resched() ||
> > vcpu_is_preempted(node_cpu(node->prev))))
> >                 return true;
> > 
> > @@ -224,11 +224,11 @@ void osq_unlock(struct optimistic_spin_queue *lock)
> >         node = this_cpu_ptr(&osq_node);
> >         next = xchg(&node->next, NULL);
> >         if (next) {
> > -               WRITE_ONCE(next->locked, 1);
> > +               smp_store_release(&next->locked, 1);
> >                 return;
> >         }
> > 
> >         next = osq_wait_next(lock, node, OSQ_UNLOCKED_VAL);
> >         if (next)
> > -               WRITE_ONCE(next->locked, 1);
> > +               smp_store_release(&next->locked, 1);
> >  }  
> 
> The test passes with the above patch:

Do you know which part matters?

> 
> # dmesg|grep mx
> [    7.010502] mx_test: osq_lock                    padding:  8 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.000 seconds
> [   12.014144] mx_test: osq_lock                    padding: 16 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.004 seconds
> [   17.017572] mx_test: osq_lock                    padding: 24 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.004 seconds
> [   22.019636] mx_test: osq_lock                    padding: 32 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.004 seconds
> [   27.022192] mx_test: osq_lock                    padding: 40 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.000 seconds
> [   32.024907] mx_test: osq_lock                    padding: 48 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.004 seconds
> [   37.026035] mx_test: osq_lock                    padding: 56 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.000 seconds
> 
> I'll use David's advise about including the osq_lock code in my test,
> so I can test it as a module, which will be more thorough.

At least with a build/run option...

> If you submit this patch, feel free to add:

I'll roll it into my patches (as 1/n).

David

> 
> Tested-by: Håkon Bugge <haakon.bugge@oracle.com>
> 
> 
> Thxs, Håkon
> 
>