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Subject[PATCH v11 4/5] locking/qspinlock: Introduce starvation avoidance into CNA
Date
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;
} else {
/* add to the tail of the secondary queue */
struct mcs_spinlock *tail_2nd = decode_tail(node->locked);
@@ -249,11 +283,15 @@ static __always_inline u32 cna_wait_head_or_lock(struct qspinlock *lock,
{
struct cna_node *cn = (struct cna_node *)node;

- /*
- * Try and put the time otherwise spent spin waiting on
- * _Q_LOCKED_PENDING_MASK to use by sorting our lists.
- */
- cn->partial_order = cna_order_queue(node);
+ if (!cn->start_time || !intra_node_threshold_reached(cn)) {
+ /*
+ * Try and put the time otherwise spent spin waiting on
+ * _Q_LOCKED_PENDING_MASK to use by sorting our lists.
+ */
+ cn->partial_order = cna_order_queue(node);
+ } else {
+ cn->partial_order = FLUSH_SECONDARY_QUEUE;
+ }

return 0; /* we lied; we didn't wait, go do so now */
}
@@ -276,13 +314,29 @@ static inline void cna_lock_handoff(struct mcs_spinlock *node,
*/
WARN_ON(partial_order == LOCAL_WAITER_NOT_FOUND);

- /*
- * We found a local waiter; reload @next in case it was changed by
- * cna_order_queue().
- */
- next = node->next;
- if (node->locked > 1)
- val = node->locked; /* preseve secondary queue */
+ if (partial_order == LOCAL_WAITER_FOUND) {
+ /*
+ * We found a local waiter; reload @next in case it
+ * was changed by cna_order_queue().
+ */
+ next = node->next;
+ if (node->locked > 1) {
+ val = node->locked; /* preseve secondary queue */
+ ((struct cna_node *)next)->start_time = cn->start_time;
+ }
+ } else {
+ WARN_ON(partial_order != FLUSH_SECONDARY_QUEUE);
+ /*
+ * We decided to flush the secondary queue;
+ * this can only happen if that queue is not empty.
+ */
+ WARN_ON(node->locked <= 1);
+ /*
+ * Splice the secondary queue onto the primary queue and pass the lock
+ * to the longest waiting remote waiter.
+ */
+ next = cna_splice_head(NULL, 0, node, next);
+ }

arch_mcs_lock_handoff(&next->locked, val);
}
@@ -334,3 +388,20 @@ void __init cna_configure_spin_lock_slowpath(void)

pr_info("Enabling CNA spinlock\n");
}
+
+static int __init numa_spinlock_threshold_setup(char *str)
+{
+ int param;
+
+ if (get_option(&str, &param)) {
+ /* valid value is between 1 and 100 */
+ if (param <= 0 || param > 100)
+ return 0;
+
+ intra_node_handoff_threshold = msecs_to_jiffies(param);
+ return 1;
+ }
+
+ return 0;
+}
+__setup("numa_spinlock_threshold=", numa_spinlock_threshold_setup);
--
2.21.1 (Apple Git-122.3)
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