linux-kernel - Re: [PATCH V2] locking/qspinlock: Optimize pending state waiting for unlock

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Date:   Wed, 4 Jan 2023 22:34:54 -0500
From:   Waiman Long <longman@...hat.com>
To:     guoren@...nel.org, peterz@...radead.org, mingo@...hat.com
Cc:     linux-kernel@...r.kernel.org, Guo Ren <guoren@...ux.alibaba.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH V2] locking/qspinlock: Optimize pending state waiting for
 unlock

On 1/4/23 21:19, 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 situation is
> the SMT scenarios in the same core. Not an entering low-power state
> situation. Of course, the granularity between cores is "cacheline",
> but the granularity between SMT hw threads of the same core could
> be "byte" which internal LSU handles. For example, when a hw-thread
> yields the resources of the core to other hw-threads, this patch
> could help the hw-thread stay in the sleep state and prevent it
> from being woken up by other hw-threads xchg_tail.
>
> Link: https://lore.kernel.org/lkml/20221224120545.262989-1-guoren@kernel.org/
> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@...nel.org>
> Acked-by: 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>
> ---
> Changes in v2:
>   - Add acked tag
>   - Optimize commit log
>   - Add discussion Link tag
> ---
>   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
> @@ -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.

Yes, the new patch description looks good to me. Thank for sending the v2.

Cheers,
Longman