linux-kernel - Re: [PATCH next 5/5] locking/osq_lock: Optimise vcpu_is

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Date: Fri, 29 Dec 2023 22:13:43 -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 5/5] locking/osq_lock: Optimise vcpu_is_preempted()
 check.


On 12/29/23 15:58, David Laight wrote:
> 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.
>
> This is simpler than checking for 'node->prev' changing and caching
> 'prev->cpu'.
>
> Signed-off-by: David Laight <david.laight@...lab.com>
> ---
>   kernel/locking/osq_lock.c | 14 ++++++--------
>   1 file changed, 6 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index b60b0add0161..89be63627434 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -14,8 +14,9 @@
>   
>   struct optimistic_spin_node {
>   	struct optimistic_spin_node *self, *next, *prev;
> -	int locked; /* 1 if lock acquired */
> -	int cpu; /* encoded CPU # + 1 value */
> +	int locked;    /* 1 if lock acquired */
> +	int cpu;       /* encoded CPU # + 1 value */
> +	int prev_cpu;  /* actual CPU # for vpcu_is_preempted() */
>   };
>   
>   static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
> @@ -29,11 +30,6 @@ static inline int encode_cpu(int cpu_nr)
>   	return cpu_nr + 1;
>   }
>   
> -static inline int node_cpu(struct optimistic_spin_node *node)
> -{
> -	return node->cpu - 1;
> -}
> -
>   static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
>   {
>   	int cpu_nr = encoded_cpu_val - 1;
> @@ -114,6 +110,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>   	if (old == OSQ_UNLOCKED_VAL)
>   		return true;
>   
> +	node->prev_cpu = old - 1;
>   	prev = decode_cpu(old);
>   	node->prev = prev;
>   	node->locked = 0;
> @@ -148,7 +145,7 @@ 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(node->prev_cpu)))
>   		return true;
>   
>   	/* unqueue */
> @@ -205,6 +202,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>   	 * it will wait in Step-A.
>   	 */
>   
> +	WRITE_ONCE(next->prev_cpu, prev->cpu - 1);
>   	WRITE_ONCE(next->prev, prev);
>   	WRITE_ONCE(prev->next, next);
Reviewed-by: Waiman Long <longman@...hat.com>

>