Re: [PATCH next v2 2/5] locking/osq_lock: Optimise the vcpu_is_preempted() check.

Waiman Long
2024-01-01T04:09:24Z

On 12/31/23 16:52, 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@aculab.com>
> ---
> kernel/locking/osq_lock.c | 16 +++++++---------
> 1 file changed, 7 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index e0bc74d85a76..eb8a6dfdb79d 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 *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; /* encoded CPU # + 1 value */
> };
>
> 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;
> @@ -110,9 +106,10 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> if (old == OSQ_UNLOCKED_VAL)
> return true;
>
> - node->locked = 0;
> + node->prev_cpu = old;
> prev = decode_cpu(old);
> node->prev = prev;
> + node->locked = 0;
>
> /*
> * osq_lock() unqueue
> @@ -144,7 +141,7 @@ 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(READ_ONCE(node->prev_cpu) - 1)))
> return true;
>
> /* unqueue */
> @@ -201,6 +198,7 @@ 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);
>

Reviewed-by: Waiman Long <longman@redhat.com>

Reviewed-by: Waiman Long <longman@redhat.com>