Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

[Waiman Long <longman@redhat.com>]

On 3/29/21 11:13 PM, Guo Ren wrote:
> On Mon, Mar 29, 2021 at 8:50 PM Peter Zijlstra <peterz@infradead.org> wrote:
>> On Mon, Mar 29, 2021 at 08:01:41PM +0800, Guo Ren wrote:
>>> u32 a = 0x55aa66bb;
>>> u16 *ptr = &a;
>>>
>>> CPU0                       CPU1
>>> =========             =========
>>> xchg16(ptr, new)     while(1)
>>>                                      WRITE_ONCE(*(ptr + 1), x);
>>>
>>> When we use lr.w/sc.w implement xchg16, it'll cause CPU0 deadlock.
>> Then I think your LL/SC is broken.
>>
>> That also means you really don't want to build super complex locking
>> primitives on top, because that live-lock will percolate through.
> Do you mean the below implementation has live-lock risk?
> +static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
> +{
> +       u32 old, new, val = atomic_read(&lock->val);
> +
> +       for (;;) {
> +               new = (val & _Q_LOCKED_PENDING_MASK) | tail;
> +               old = atomic_cmpxchg(&lock->val, val, new);
> +               if (old == val)
> +                       break;
> +
> +               val = old;
> +       }
> +       return old;
> +}
If there is a continuous stream of incoming spinlock takers, it is 
possible that some cpus may have to wait a long time to set the tail 
right. However, this should only happen on artificial workload. I doubt 
it will happen with real workload or with limit number of cpus.
>
>> Step 1 would be to get your architecute fixed such that it can provide
>> fwd progress guarantees for LL/SC. Otherwise there's absolutely no point
>> in building complex systems with it.
> Quote Waiman's comment [1] on xchg16 optimization:
>
> "This optimization is needed to make the qspinlock achieve performance
> parity with ticket spinlock at light load."
>
> [1] https://lore.kernel.org/kvm/1429901803-29771-6-git-send-email-Waiman.Long@hp.com/
>
> So for a non-xhg16 machine:
>   - ticket-lock for small numbers of CPUs
>   - qspinlock for large numbers of CPUs
>
> Okay, I'll put all of them into the next patch :P
>
It is true that qspinlock may not offer much advantage when the number 
of cpus is small. It shines for systems with many cpus. You may use 
NR_CPUS to determine if the default should be ticket or qspinlock with 
user override. To determine the right NR_CPUS threshold, you may need to 
run on real SMP RISCV systems to find out.

Cheers,
Longman