lkml.org 
[lkml]   [2020]   [Sep]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH v11 4/5] locking/qspinlock: Introduce starvation avoidance into CNA
From
Date
On 9/15/20 2:05 PM, Alex Kogan wrote:
> Keep track of the time the thread at the head of the secondary queue
> has been waiting, and force inter-node handoff once this time passes
> a preset threshold. The default value for the threshold (10ms) can be
> overridden with the new kernel boot command-line option
> "numa_spinlock_threshold". The ms value is translated internally to the
> nearest rounded-up jiffies.
>
> Signed-off-by: Alex Kogan <alex.kogan@oracle.com>
> Reviewed-by: Steve Sistare <steven.sistare@oracle.com>
> Reviewed-by: Waiman Long <longman@redhat.com>
> ---
> .../admin-guide/kernel-parameters.txt | 9 ++
> kernel/locking/qspinlock_cna.h | 95 ++++++++++++++++---
> 2 files changed, 92 insertions(+), 12 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 51ce050f8701..73ab23a47b97 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3363,6 +3363,15 @@
> Not specifying this option is equivalent to
> numa_spinlock=auto.
>
> + numa_spinlock_threshold= [NUMA, PV_OPS]
> + Set the time threshold in milliseconds 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 [1..100] range. Smaller values result
> + in a more fair, but less performant spinlock,
> + and vice versa. The default value is 10.
> +
> 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_cna.h b/kernel/locking/qspinlock_cna.h
> index 590402ad69ef..d3e27549c769 100644
> --- a/kernel/locking/qspinlock_cna.h
> +++ b/kernel/locking/qspinlock_cna.h
> @@ -37,6 +37,12 @@
> * gradually filter the primary queue, leaving only waiters running on the same
> * preferred NUMA node.
> *
> + * We change the NUMA node preference after a waiter at the head of the
> + * secondary queue spins for a certain amount of time (10ms, by default).
> + * We do that by flushing the secondary queue into the head of the primary queue,
> + * effectively changing the preference to the NUMA node of the waiter at the head
> + * of the secondary queue at the time of the flush.
> + *
> * For more details, see https://arxiv.org/abs/1810.05600.
> *
> * Authors: Alex Kogan <alex.kogan@oracle.com>
> @@ -49,13 +55,33 @@ struct cna_node {
> u16 real_numa_node;
> u32 encoded_tail; /* self */
> u32 partial_order; /* enum val */
> + s32 start_time;
> };
>
> enum {
> LOCAL_WAITER_FOUND,
> LOCAL_WAITER_NOT_FOUND,
> + FLUSH_SECONDARY_QUEUE
> };
>
> +/*
> + * Controls the threshold time in ms (default = 10) for intra-node lock
> + * hand-offs before the NUMA-aware variant of spinlock is forced to be
> + * passed to a thread on another NUMA node. The default setting can be
> + * changed with the "numa_spinlock_threshold" boot option.
> + */
> +#define MSECS_TO_JIFFIES(m) \
> + (((m) + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ))
> +static int intra_node_handoff_threshold __ro_after_init = MSECS_TO_JIFFIES(10);
> +
> +static inline bool intra_node_threshold_reached(struct cna_node *cn)
> +{
> + s32 current_time = (s32)jiffies;
> + s32 threshold = cn->start_time + intra_node_handoff_threshold;
> +
> + return current_time - threshold > 0;
> +}
> +
> static void __init cna_init_nodes_per_cpu(unsigned int cpu)
> {
> struct mcs_spinlock *base = per_cpu_ptr(&qnodes[0].mcs, cpu);
> @@ -98,6 +124,7 @@ static __always_inline void cna_init_node(struct mcs_spinlock *node)
> struct cna_node *cn = (struct cna_node *)node;
>
> cn->numa_node = cn->real_numa_node;
> + cn->start_time = 0;
> }
>
> /*
> @@ -197,8 +224,15 @@ static void cna_splice_next(struct mcs_spinlock *node,
>
> /* stick `next` on the secondary queue tail */
> if (node->locked <= 1) { /* if secondary queue is empty */
> + struct cna_node *cn = (struct cna_node *)node;
> +
> /* create secondary queue */
> next->next = next;
> +
> + cn->start_time = (s32)jiffies;
> + /* make sure start_time != 0 iff secondary queue is not empty */
> + if (!cn->start_time)
> + cn->start_time = 1;

My version of the patch set the end_time while yours set the start_time.
The differences are

1) When the node time is 0, yours will reduce the jiffies count by 1. My
version will increase it by 1.
2) The intra_node_handoff_threshold is accessed here in my version when
setting the end time while yours will access it in
intra_node_threshold_reached().

It is largely a matter of preference, but I am just curious about what
you think about the advantage of doing it this way.

Cheers,
Longman

\
 
 \ /
  Last update: 2020-09-16 05:14    [W:0.073 / U:0.596 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site