linux-kernel - Re: [PATCH] locking/qspinlock: Optimize pending state waiting for unlock

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Date:   Sat, 24 Dec 2022 20:55:09 -0500
From:   Waiman Long <longman@...hat.com>
To:     guoren@...nel.org, peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org, Guo Ren <guoren@...ux.alibaba.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] locking/qspinlock: Optimize pending state waiting for
 unlock

On 12/24/22 07:05, guoren@...nel.org wrote:
> From: Guo Ren <guoren@...ux.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@...ux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@...nel.org>
> Cc: Waiman Long <longman@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Ingo Molnar <mingo@...hat.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