Re: [PATCH v3 next 2/5] Optimise vcpu_is_preempted() check

[David Laight <david.laight.linux@gmail.com>]

On Fri,  6 Mar 2026 22:51:47 +0000
david.laight.linux@gmail.com wrote:

Apologies to Yafang for mistyping his address...

> From: David Laight <david.laight.linux@gmail.com>
> 
> The vcpu_is_preempted() test stops osq_lock() spinning if a virtual
>   CPU is no longer running.
> Although patched out for bare-metal, the code still needs the CPU number.
> Reading this from 'prev->cpu' is a pretty much guaranteed have a cache miss
> when osq_unlock() is waking up the next cpu.
> 
> Instead save 'prev->cpu' in 'node->prev_cpu' and use that value instead.
> Update in the osq_lock() 'unqueue' path when 'node->prev' is changed.
> 
> Signed-off-by: David Laight <david.laight.linux@gmail.com>
> ---
>  kernel/locking/osq_lock.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 96c6094157b5..0e1c7d11b6c0 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -16,6 +16,7 @@ struct optimistic_spin_node {
>  	struct optimistic_spin_node *next, *prev;
>  	int locked; /* 1 if lock acquired */
>  	int cpu; /* encoded CPU # + 1 value */
> +	int prev_cpu; /* encoded CPU # + 1 value */
>  };
>  
>  static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
> @@ -29,9 +30,9 @@ static inline int encode_cpu(int cpu_nr)
>  	return cpu_nr + 1;
>  }
>  
> -static inline int node_cpu(struct optimistic_spin_node *node)
> +static inline int prev_cpu_nr(struct optimistic_spin_node *node)
>  {
> -	return node->cpu - 1;
> +	return READ_ONCE(node->prev_cpu) - 1;
>  }
>  
>  static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
> @@ -110,6 +111,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>  	if (old == OSQ_UNLOCKED_VAL)
>  		return true;
>  
> +	WRITE_ONCE(node->prev_cpu, old);
>  	prev = decode_cpu(old);
>  	node->prev = prev;
>  	node->locked = 0;
> @@ -144,7 +146,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>  	 * polling, be careful.
>  	 */
>  	if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
> -				  vcpu_is_preempted(node_cpu(node->prev))))
> +				  vcpu_is_preempted(prev_cpu_nr(node))))
>  		return true;
>  
>  	/* unqueue */
> @@ -201,6 +203,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>  	 * it will wait in Step-A.
>  	 */
>  
> +	WRITE_ONCE(next->prev_cpu, prev->cpu);
>  	WRITE_ONCE(next->prev, prev);
>  	WRITE_ONCE(prev->next, next);
>