linux-kernel - Re: [PATCH next 2/5] locking/osq_lock: Avoid dirtying the local cpu's 'node' in the osq

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Date: Fri, 29 Dec 2023 22:20:25 -0500
From: Waiman Long <longman@...hat.com>
To: David Laight <David.Laight@...LAB.COM>,
 "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
 "'peterz@...radead.org'" <peterz@...radead.org>
Cc: "'mingo@...hat.com'" <mingo@...hat.com>,
 "'will@...nel.org'" <will@...nel.org>,
 "'boqun.feng@...il.com'" <boqun.feng@...il.com>,
 'Linus Torvalds' <torvalds@...ux-foundation.org>,
 "'xinhui.pan@...ux.vnet.ibm.com'" <xinhui.pan@...ux.vnet.ibm.com>,
 "'virtualization@...ts.linux-foundation.org'"
 <virtualization@...ts.linux-foundation.org>,
 'Zeng Heng' <zengheng4@...wei.com>
Subject: Re: [PATCH next 2/5] locking/osq_lock: Avoid dirtying the local cpu's
 'node' in the osq_lock() fast path.

On 12/29/23 17:11, David Laight wrote:
> osq_lock() starts by setting node->next to NULL and node->locked to 0.
> Careful analysis shows that node->next is always NULL on entry.
>
> node->locked is set non-zero by another cpu to force a wakeup.
> This can only happen after the 'prev->next = node' assignment,
> so locked can be set to zero just before that (along with the assignment
> to node->prev).
>
> Only initialise node->cpu once, after that use its value instead
> of smp_processor_id() - which is probably a real function call.
>
> Should reduce cache-line bouncing a little.
>
> Signed-off-by: David Laight <david.laight@...lab.com>
> ---
>
> Re-send without the 'RE:' on the subject line.
>   kernel/locking/osq_lock.c | 13 ++++++-------
>   1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index d414eef4bec6..55f5db896c02 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -51,7 +51,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
>   	      struct optimistic_spin_node *prev)
>   {
>   	struct optimistic_spin_node *next = NULL;
> -	int curr = encode_cpu(smp_processor_id());
> +	int curr = node->cpu;
>   	int old;
>   
>   	/*
> @@ -98,12 +98,10 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>   {
>   	struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
>   	struct optimistic_spin_node *prev, *next;
> -	int curr = encode_cpu(smp_processor_id());
>   	int old;
>   
> -	node->locked = 0;
> -	node->next = NULL;

I have some concern about not clearing node->next at the beginning. I 
know that next is supposed to be cleared at the end. I do have some 
worry that there may exist a race condition that leaves next not cleared 
before it is used again. So you either have to prove that such a race 
does not exist, or initializing it to NULL at the beginning like it is 
today.

Cheers,
Longman

> -	node->cpu = curr;
> +	if (unlikely(node->cpu == OSQ_UNLOCKED_VAL))
> +		node->cpu = encode_cpu(smp_processor_id());
>   
>   	/*
>   	 * We need both ACQUIRE (pairs with corresponding RELEASE in
> @@ -111,12 +109,13 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>   	 * the node fields we just initialised) semantics when updating
>   	 * the lock tail.
>   	 */
> -	old = atomic_xchg(&lock->tail, curr);
> +	old = atomic_xchg(&lock->tail, node->cpu);
>   	if (old == OSQ_UNLOCKED_VAL)
>   		return true;
>   
>   	prev = decode_cpu(old);
>   	node->prev = prev;
> +	node->locked = 0;
>   
>   	/*
>   	 * osq_lock()			unqueue
> @@ -214,7 +213,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>   void osq_unlock(struct optimistic_spin_queue *lock)
>   {
>   	struct optimistic_spin_node *node, *next;
> -	int curr = encode_cpu(smp_processor_id());
> +	int curr = raw_cpu_read(osq_node.cpu);
>   
>   	/*
>   	 * Fast path for the uncontended case.