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SubjectRe: [PATCH] locking/qspinlock: Optimize pending state waiting for unlock
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On 12/24/22 07:05, guoren@kernel.org wrote:
> From: Guo Ren <guoren@linux.alibaba.com>
>
> When we're pending, we only care about lock value. The xchg_tail
> wouldn't affect the pending state. That means the hardware thread
> could stay in a sleep state and leaves the rest execution units'
> resources of pipeline to other hardware threads. This optimization
> may work only for SMT scenarios because the granularity between
> cores is cache-block.
>
> Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@kernel.org>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> ---
> kernel/locking/qspinlock.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 2b23378775fe..ebe6b8ec7cb3 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -371,7 +371,7 @@ void __lockfunc queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> /*
> * We're pending, wait for the owner to go away.
> *
> - * 0,1,1 -> 0,1,0
> + * 0,1,1 -> *,1,0
> *
> * this wait loop must be a load-acquire such that we match the
> * store-release that clears the locked bit and create lock
Yes, we don't care about the tail.
> @@ -380,7 +380,7 @@ void __lockfunc queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> * barriers.
> */
> if (val & _Q_LOCKED_MASK)
> - atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_MASK));
> + smp_cond_load_acquire(&lock->locked, !VAL);
>
> /*
> * take ownership and clear the pending bit.

We may save an AND operation here which may be a cycle or two.  I
remember that it may be more costly to load a byte instead of an integer
in some arches. So it doesn't seem like that much of an optimization
from my point of view. I know that arm64 will enter a low power state in
this *cond_load_acquire() loop, but I believe any change in the state of
the the lock cacheline will wake it up. So it doesn't really matter if
you are checking a byte or an int.

Do you have any other data point to support your optimization claim?

Cheers,
Longman

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