[PATCH 8/9] qspinlock: Generic paravirt support

[Peter Zijlstra]

Implement simple paravirt support for the qspinlock.

Provide a separate (second) version of the spin_lock_slowpath for
paravirt along with a special unlock path.

The second slowpath is generated by adding a few pv hooks to the
normal slowpath, but where those will compile away for the native
case, they expand into special wait/wake code for the pv version.

The actual MCS queue can use extra storage in the mcs_nodes[] array to
keep track of state and therefore uses directed wakeups.

The head contender has no such storage available and reverts to the
per-cpu lock entry similar to the current kvm code. We can do a single
enrty because any nesting will wake the vcpu and cause the lower loop
to retry.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 include/asm-generic/qspinlock.h     |    3 
 kernel/locking/qspinlock.c          |   69 +++++++++++++-
 kernel/locking/qspinlock_paravirt.h |  177 ++++++++++++++++++++++++++++++++++++
 3 files changed, 248 insertions(+), 1 deletion(-)

--- a/include/asm-generic/qspinlock.h
+++ b/include/asm-generic/qspinlock.h
@@ -118,6 +118,9 @@ static __always_inline bool virt_queue_s
 }
 #endif
 
+extern void __pv_queue_spin_lock_slowpath(struct qspinlock *lock, u32 val);
+extern void __pv_queue_spin_unlock(struct qspinlock *lock);
+
 /*
  * Initializier
  */
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -18,6 +18,9 @@
  * Authors: Waiman Long <waiman.long@xxxxxx>
  *          Peter Zijlstra <peterz@xxxxxxxxxxxxx>
  */
+
+#ifndef _GEN_PV_LOCK_SLOWPATH
+
 #include <linux/smp.h>
 #include <linux/bug.h>
 #include <linux/cpumask.h>
@@ -65,13 +68,21 @@
 
 #include "mcs_spinlock.h"
 
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+#define MAX_NODES	8
+#else
+#define MAX_NODES	4
+#endif
+
 /*
  * Per-CPU queue node structures; we can never have more than 4 nested
  * contexts: task, softirq, hardirq, nmi.
  *
  * Exactly fits one 64-byte cacheline on a 64-bit architecture.
+ *
+ * PV doubles the storage and uses the second cacheline for PV state.
  */
-static DEFINE_PER_CPU_ALIGNED(struct mcs_spinlock, mcs_nodes[4]);
+static DEFINE_PER_CPU_ALIGNED(struct mcs_spinlock, mcs_nodes[MAX_NODES]);
 
 /*
  * We must be able to distinguish between no-tail and the tail at 0:0,
@@ -230,6 +241,32 @@ static __always_inline void set_locked(s
 	WRITE_ONCE(l->locked, _Q_LOCKED_VAL);
 }
 
+
+/*
+ * Generate the native code for queue_spin_unlock_slowpath(); provide NOPs for
+ * all the PV callbacks.
+ */
+
+static __always_inline void __pv_init_node(struct mcs_spinlock *node) { }
+static __always_inline void __pv_wait_node(struct mcs_spinlock *node) { }
+static __always_inline void __pv_kick_node(struct mcs_spinlock *node) { }
+
+static __always_inline void __pv_wait_head(struct qspinlock *lock) { }
+
+#define pv_enabled()		false
+
+#define pv_init_node		__pv_init_node
+#define pv_wait_node		__pv_wait_node
+#define pv_kick_node		__pv_kick_node
+
+#define pv_wait_head		__pv_wait_head
+
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+#define queue_spin_lock_slowpath	native_queue_spin_lock_slowpath
+#endif
+
+#endif /* _GEN_PV_LOCK_SLOWPATH */
+
 /**
  * queue_spin_lock_slowpath - acquire the queue spinlock
  * @lock: Pointer to queue spinlock structure
@@ -259,6 +296,9 @@ void queue_spin_lock_slowpath(struct qsp
 
 	BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
 
+	if (pv_enabled())
+		goto queue;
+
 	if (virt_queue_spin_lock(lock))
 		return;
 
@@ -335,6 +375,7 @@ void queue_spin_lock_slowpath(struct qsp
 	node += idx;
 	node->locked = 0;
 	node->next = NULL;
+	pv_init_node(node);
 
 	/*
 	 * We touched a (possibly) cold cacheline in the per-cpu queue node;
@@ -360,6 +401,7 @@ void queue_spin_lock_slowpath(struct qsp
 		prev = decode_tail(old);
 		WRITE_ONCE(prev->next, node);
 
+		pv_wait_node(node);
 		arch_mcs_spin_lock_contended(&node->locked);
 	}
 
@@ -374,6 +416,7 @@ void queue_spin_lock_slowpath(struct qsp
 	 * sequentiality; this is because the set_locked() function below
 	 * does not imply a full barrier.
 	 */
+	pv_wait_head(lock);
 	while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK)
 		cpu_relax();
 
@@ -406,6 +449,7 @@ void queue_spin_lock_slowpath(struct qsp
 		cpu_relax();
 
 	arch_mcs_spin_unlock_contended(&next->locked);
+	pv_kick_node(next);
 
 release:
 	/*
@@ -414,3 +458,26 @@ void queue_spin_lock_slowpath(struct qsp
 	this_cpu_dec(mcs_nodes[0].count);
 }
 EXPORT_SYMBOL(queue_spin_lock_slowpath);
+
+/*
+ * Generate the paravirt code for queue_spin_unlock_slowpath().
+ */
+#if !defined(_GEN_PV_LOCK_SLOWPATH) && defined(CONFIG_PARAVIRT_SPINLOCKS)
+#define _GEN_PV_LOCK_SLOWPATH
+
+#undef pv_enabled
+#define pv_enabled()	true
+
+#undef pv_init_node
+#undef pv_wait_node
+#undef pv_kick_node
+
+#undef pv_wait_head
+
+#undef queue_spin_lock_slowpath
+#define queue_spin_lock_slowpath	__pv_queue_spin_lock_slowpath
+
+#include "qspinlock_paravirt.h"
+#include "qspinlock.c"
+
+#endif
--- /dev/null
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -0,0 +1,177 @@
+#ifndef _GEN_PV_LOCK_SLOWPATH
+#error "do not include this file"
+#endif
+
+/*
+ * Implement paravirt qspinlocks; the general idea is to halt the vcpus instead
+ * of spinning them.
+ *
+ * This relies on the architecture to provide two paravirt hypercalls:
+ *
+ *   pv_wait(u8 *ptr, u8 val) -- suspends the vcpu if *ptr == val
+ *   pv_kick(cpu)             -- wakes a suspended vcpu
+ *
+ * Using these we implement __pv_queue_spin_lock_slowpath() and
+ * __pv_queue_spin_unlock() to replace native_queue_spin_lock_slowpath() and
+ * native_queue_spin_unlock().
+ */
+
+#define _Q_SLOW_VAL	(2U << _Q_LOCKED_OFFSET)
+
+enum vcpu_state {
+	vcpu_running = 0,
+	vcpu_halted,
+};
+
+struct pv_node {
+	struct mcs_spinlock	mcs;
+	struct mcs_spinlock	__res[3];
+
+	int			cpu;
+	u8			state;
+};
+
+/*
+ * Initialize the PV part of the mcs_spinlock node.
+ */
+static void pv_init_node(struct mcs_spinlock *node)
+{
+	struct pv_node *pn = (struct pv_node *)node;
+
+	BUILD_BUG_ON(sizeof(struct pv_node) > 5*sizeof(struct mcs_spinlock));
+
+	pn->cpu = smp_processor_id();
+	pn->state = vcpu_running;
+}
+
+/*
+ * Wait for node->locked to become true, halt the vcpu after a short spin.
+ * pv_kick_node() is used to wake the vcpu again.
+ */
+static void pv_wait_node(struct mcs_spinlock *node)
+{
+	struct pv_node *pn = (struct pv_node *)node;
+	int loop;
+
+	for (;;) {
+		for (loop = SPIN_THRESHOLD; loop; loop--) {
+			if (READ_ONCE(node->locked))
+				goto done;
+
+			cpu_relax();
+		}
+
+		/*
+		 * Order pn->state vs pn->locked thusly:
+		 *
+		 * [S] pn->state = vcpu_halted	  [S] next->locked = 1
+		 *     MB			      MB
+		 * [L] pn->locked		[RmW] pn->state = vcpu_running
+		 *
+		 * Matches the xchg() from pv_kick_node().
+		 */
+		(void)xchg(&pn->state, vcpu_halted);
+
+		if (READ_ONCE(node->locked))
+			goto done;
+
+		pv_wait(&pn->state, vcpu_halted);
+	}
+done:
+	pn->state = vcpu_running;
+
+	/*
+	 * By now our node->locked should be 1 and our caller will not actually
+	 * spin-wait for it. We do however rely on our caller to do a
+	 * load-acquire for us.
+	 */
+}
+
+/*
+ * Called after setting next->locked = 1, used to wake those stuck in
+ * pv_wait_node().
+ */
+static void pv_kick_node(struct mcs_spinlock *node)
+{
+	struct pv_node *pn = (struct pv_node *)node;
+
+	/*
+	 * Note that because node->locked is already set, this actual mcs_spinlock
+	 * entry could be re-used already.
+	 *
+	 * This should be fine however, kicking people for no reason is harmless.
+	 *
+	 * See the comment in pv_wait_node().
+	 */
+	if (xchg(&pn->state, vcpu_running) == vcpu_halted)
+		pv_kick(pn->cpu);
+}
+
+static DEFINE_PER_CPU(struct qspinlock *, __pv_lock_wait);
+
+/*
+ * Wait for l->locked to become clear; halt the vcpu after a short spin.
+ * __pv_queue_spin_unlock() will wake us.
+ */
+static void pv_wait_head(struct qspinlock *lock)
+{
+	struct __qspinlock *l = (void *)lock;
+	int loop;
+
+	for (;;) {
+		for (loop = SPIN_THRESHOLD; loop; loop--) {
+			if (!READ_ONCE(l->locked))
+				goto done;
+
+			cpu_relax();
+		}
+
+		this_cpu_write(__pv_lock_wait, lock);
+		/*
+		 * __pv_lock_wait must be set before setting _Q_SLOW_VAL
+		 *
+		 * [S] __pv_lock_wait = lock    [RmW] l = l->locked = 0
+		 *     MB                             MB
+		 * [S] l->locked = _Q_SLOW_VAL  [L]   __pv_lock_wait
+		 *
+		 * Matches the xchg() in pv_queue_spin_unlock().
+		 */
+		if (!cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL))
+			goto done;
+
+		pv_wait(&l->locked, _Q_SLOW_VAL);
+	}
+done:
+	this_cpu_write(__pv_lock_wait, NULL);
+
+	/*
+	 * Lock is unlocked now; the caller will acquire it without waiting.
+	 * As with pv_wait_node() we rely on the caller to do a load-acquire
+	 * for us.
+	 */
+}
+
+/*
+ * To be used in stead of queue_spin_unlock() for paravirt locks. Wakes
+ * pv_wait_head() if appropriate.
+ */
+void __pv_queue_spin_unlock(struct qspinlock *lock)
+{
+	struct __qspinlock *l = (void *)lock;
+	int cpu;
+
+	if (xchg(&l->locked, 0) != _Q_SLOW_VAL)
+		return;
+
+	/*
+	 * At this point the memory pointed at by lock can be freed/reused,
+	 * however we can still use the pointer value to search in our cpu
+	 * array.
+	 *
+	 * XXX: get rid of this loop
+	 */
+	for_each_possible_cpu(cpu) {
+		if (per_cpu(__pv_lock_wait, cpu) == lock)
+			pv_kick(cpu);
+	}
+}


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