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Message-Id: <20210514200743.3026725-5-alex.kogan@oracle.com>
Date: Fri, 14 May 2021 16:07:41 -0400
From: Alex Kogan <alex.kogan@...cle.com>
To: linux@...linux.org.uk, peterz@...radead.org, mingo@...hat.com,
will.deacon@....com, arnd@...db.de, longman@...hat.com,
linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org, tglx@...utronix.de, bp@...en8.de,
hpa@...or.com, x86@...nel.org, guohanjun@...wei.com,
jglauber@...vell.com
Cc: steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
alex.kogan@...cle.com, dave.dice@...cle.com
Subject: [PATCH v15 4/6] locking/qspinlock: Introduce starvation avoidance into CNA
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 (1ms) can be
overridden with the new kernel boot command-line option
"qspinlock.numa_spinlock_threshold_ns".
Signed-off-by: Alex Kogan <alex.kogan@...cle.com>
Reviewed-by: Steve Sistare <steven.sistare@...cle.com>
Reviewed-by: Waiman Long <longman@...hat.com>
---
.../admin-guide/kernel-parameters.txt | 8 ++
kernel/locking/qspinlock_cna.h | 81 +++++++++++++++----
2 files changed, 73 insertions(+), 16 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 94d35507560c..e2bcf6f4e048 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4183,6 +4183,14 @@
[KNL] Number of legacy pty's. Overwrites compiled-in
default number.
+ qspinlock.numa_spinlock_threshold_ns= [NUMA, PV_OPS]
+ Set the time threshold in nanoseconds 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. Smaller values
+ result in a more fair, but less performant spinlock,
+ and vice versa. The default value is 1000000 (=1ms).
+
quiet [KNL] Disable most log messages
r128= [HW,DRM]
diff --git a/kernel/locking/qspinlock_cna.h b/kernel/locking/qspinlock_cna.h
index ca564e64e5de..0b991c340fb1 100644
--- a/kernel/locking/qspinlock_cna.h
+++ b/kernel/locking/qspinlock_cna.h
@@ -4,6 +4,8 @@
#endif
#include <linux/topology.h>
+#include <linux/sched/clock.h>
+#include <linux/moduleparam.h>
/*
* Implement a NUMA-aware version of MCS (aka CNA, or compact NUMA-aware lock).
@@ -37,19 +39,39 @@
* 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 (1ms, 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@...cle.com>
* Dave Dice <dave.dice@...cle.com>
*/
+#define FLUSH_SECONDARY_QUEUE 1
+
struct cna_node {
struct mcs_spinlock mcs;
u16 numa_node;
u16 real_numa_node;
u32 encoded_tail; /* self */
+ u64 start_time;
};
+static ulong numa_spinlock_threshold_ns = 1000000; /* 1ms, by default */
+module_param(numa_spinlock_threshold_ns, ulong, 0644);
+
+static inline bool intra_node_threshold_reached(struct cna_node *cn)
+{
+ u64 current_time = local_clock();
+ u64 threshold = cn->start_time + numa_spinlock_threshold_ns;
+
+ return current_time > threshold;
+}
+
static void __init cna_init_nodes_per_cpu(unsigned int cpu)
{
struct mcs_spinlock *base = per_cpu_ptr(&qnodes[0].mcs, cpu);
@@ -92,6 +114,7 @@ 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;
}
/*
@@ -191,8 +214,14 @@ 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 = local_clock();
+ /* secondary queue is not empty iff start_time != 0 */
+ WARN_ON(!cn->start_time);
} else {
/* add to the tail of the secondary queue */
struct mcs_spinlock *tail_2nd = decode_tail(node->locked);
@@ -240,12 +269,18 @@ static int cna_order_queue(struct mcs_spinlock *node)
static __always_inline u32 cna_wait_head_or_lock(struct qspinlock *lock,
struct mcs_spinlock *node)
{
- /*
- * Try and put the time otherwise spent spin waiting on
- * _Q_LOCKED_PENDING_MASK to use by sorting our lists.
- */
- while (LOCK_IS_BUSY(lock) && !cna_order_queue(node))
- cpu_relax();
+ struct cna_node *cn = (struct cna_node *)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.
+ */
+ while (LOCK_IS_BUSY(lock) && !cna_order_queue(node))
+ cpu_relax();
+ } else {
+ cn->start_time = FLUSH_SECONDARY_QUEUE;
+ }
return 0; /* we lied; we didn't wait, go do so now */
}
@@ -253,24 +288,38 @@ static __always_inline u32 cna_wait_head_or_lock(struct qspinlock *lock,
static inline void cna_lock_handoff(struct mcs_spinlock *node,
struct mcs_spinlock *next)
{
+ struct cna_node *cn = (struct cna_node *)node;
u32 val = 1;
- if (node->locked > 1) {
- struct cna_node *cn = (struct cna_node *)node;
+ if (cn->start_time != FLUSH_SECONDARY_QUEUE) {
+ if (node->locked > 1) {
+ val = node->locked; /* preseve secondary queue */
+
+ /*
+ * We have a local waiter, either real or fake one;
+ * reload @next in case it was changed by cna_order_queue().
+ */
+ next = node->next;
- val = node->locked; /* preseve secondary queue */
+ /*
+ * Pass over NUMA node id of primary queue, to maintain the
+ * preference even if the next waiter is on a different node.
+ */
+ ((struct cna_node *)next)->numa_node = cn->numa_node;
+ ((struct cna_node *)next)->start_time = cn->start_time;
+ }
+ } else {
/*
- * We have a local waiter, either real or fake one;
- * reload @next in case it was changed by cna_order_queue().
+ * We decided to flush the secondary queue;
+ * this can only happen if that queue is not empty.
*/
- next = node->next;
-
+ WARN_ON(node->locked <= 1);
/*
- * Pass over NUMA node id of primary queue, to maintain the
- * preference even if the next waiter is on a different node.
+ * Splice the secondary queue onto the primary queue and pass the lock
+ * to the longest waiting remote waiter.
*/
- ((struct cna_node *)next)->numa_node = cn->numa_node;
+ next = cna_splice_head(NULL, 0, node, next);
}
arch_mcs_lock_handoff(&next->locked, val);
--
2.24.3 (Apple Git-128)
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