Re: [PATCH v5 4/5] locking/qspinlock: Introduce starvation avoidance into CNA

Waiman Long
2019-10-18T16:09:31Z

On 10/16/19 12:29 AM, Alex Kogan wrote:
> Keep track of the number of intra-node lock handoffs, and force
> inter-node handoff once this number reaches a preset threshold.
>
> Signed-off-by: Alex Kogan <alex.kogan@oracle.com>
> Reviewed-by: Steve Sistare <steven.sistare@oracle.com>
> ---
> kernel/locking/qspinlock.c | 3 +++
> kernel/locking/qspinlock_cna.h | 30 +++++++++++++++++++++++++++---
> 2 files changed, 30 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 6d8c4a52e44e..1d0d884308ef 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -597,6 +597,9 @@ EXPORT_SYMBOL(queued_spin_lock_slowpath);
> #if !defined(_GEN_CNA_LOCK_SLOWPATH) && defined(CONFIG_NUMA_AWARE_SPINLOCKS)
> #define _GEN_CNA_LOCK_SLOWPATH
>
> +#undef pv_init_node
> +#define pv_init_node cna_init_node
> +
> #undef pv_wait_head_or_lock
> #define pv_wait_head_or_lock cna_pre_scan
>
> diff --git a/kernel/locking/qspinlock_cna.h b/kernel/locking/qspinlock_cna.h
> index 4d095f742d31..b92a6f9a19db 100644
> --- a/kernel/locking/qspinlock_cna.h
> +++ b/kernel/locking/qspinlock_cna.h
> @@ -50,9 +50,19 @@ struct cna_node {
> struct mcs_spinlock mcs;
> int numa_node;
> u32 encoded_tail;
> - u32 pre_scan_result; /* 0 or an encoded tail */
> + u32 pre_scan_result; /* 0, 1 or an encoded tail */
> + u32 intra_count;
> };
>
> +/*
> + * Controls the threshold for the number of intra-node lock hand-offs. It can
> + * be tuned and depend, e.g., on the number of CPUs per node. For now,
> + * choose a value that provides reasonable long-term fairness without
> + * sacrificing performance compared to a version that does not have any
> + * fairness guarantees.
> + */
> +#define INTRA_NODE_HANDOFF_THRESHOLD (1 << 16)

I think 64k is too high. I will be more comfortable with a number like
(1 << 8). The worst case latency for a lock waiter from the other node
is just not acceptable.

Cheers,
Longman