Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance into CNA

Waiman Long
2020-01-06T15:33:27Z

On 12/30/19 2:40 PM, 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.
> The default value for the threshold can be overridden with
> the new kernel boot command-line option "numa_spinlock_threshold".
>
> Signed-off-by: Alex Kogan <alex.kogan@oracle.com>
> Reviewed-by: Steve Sistare <steven.sistare@oracle.com>
> ---
> .../admin-guide/kernel-parameters.txt | 8 ++++
> kernel/locking/qspinlock.c | 3 ++
> kernel/locking/qspinlock_cna.h | 41 ++++++++++++++++++-
> 3 files changed, 51 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index b68cb80e477f..30d79819a3b0 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3200,6 +3200,14 @@
> Not specifying this option is equivalent to
> numa_spinlock=auto.
>
> + numa_spinlock_threshold= [NUMA, PV_OPS]
> + Set the threshold for the number of intra-node
> + lock hand-offs before the NUMA-aware spinlock
> + is forced to be passed to a thread on another NUMA node.
> + Valid values are in the [0..31] range. Smaller values
> + result in a more fair, but less performant spinlock, and
> + vice versa. The default value is 16.
> +
> cpu0_hotplug [X86] Turn on CPU0 hotplug feature when
> CONFIG_BOOTPARAM_HOTPLUG_CPU0 is off.
> Some features depend on CPU0. Known dependencies are:
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 609980a53841..e382d8946ccc 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 3c99a4a6184b..30feff02865d 100644
> --- a/kernel/locking/qspinlock_cna.h
> +++ b/kernel/locking/qspinlock_cna.h
> @@ -51,13 +51,25 @@ struct cna_node {
> int numa_node;
> u32 encoded_tail;
> u32 pre_scan_result; /* encoded tail or enum val */
> + u32 intra_count;
> };
>
> enum {
> LOCAL_WAITER_FOUND = 2, /* 0 and 1 are reserved for @locked */
> + FLUSH_SECONDARY_QUEUE = 3,
> MIN_ENCODED_TAIL
> };
>
> +/*
> + * Controls the threshold for the number of intra-node lock hand-offs before
> + * the NUMA-aware variant of spinlock is forced to be passed to a thread on
> + * another NUMA node. By default, the chosen value provides reasonable
> + * long-term fairness without sacrificing performance compared to a lock
> + * that does not have any fairness guarantees. The default setting can
> + * be changed with the "numa_spinlock_threshold" boot option.
> + */
> +int intra_node_handoff_threshold __ro_after_init = 1 << 16;

Another minor nit. (1 << 31) is negative for an int value. I will
suggest that you either make intra_node_handoff_threshold an unsigned
int or limit the upper bound to 30.

Cheers,
Longman