summaryrefslogtreecommitdiffstats
path: root/kernel/locking/qspinlock_paravirt.h
diff options
context:
space:
mode:
Diffstat (limited to 'kernel/locking/qspinlock_paravirt.h')
-rw-r--r--kernel/locking/qspinlock_paravirt.h562
1 files changed, 562 insertions, 0 deletions
diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
new file mode 100644
index 000000000..6afc249ce
--- /dev/null
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -0,0 +1,562 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _GEN_PV_LOCK_SLOWPATH
+#error "do not include this file"
+#endif
+
+#include <linux/hash.h>
+#include <linux/memblock.h>
+#include <linux/debug_locks.h>
+
+/*
+ * 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_queued_spin_lock_slowpath() and
+ * __pv_queued_spin_unlock() to replace native_queued_spin_lock_slowpath() and
+ * native_queued_spin_unlock().
+ */
+
+#define _Q_SLOW_VAL (3U << _Q_LOCKED_OFFSET)
+
+/*
+ * Queue Node Adaptive Spinning
+ *
+ * A queue node vCPU will stop spinning if the vCPU in the previous node is
+ * not running. The one lock stealing attempt allowed at slowpath entry
+ * mitigates the slight slowdown for non-overcommitted guest with this
+ * aggressive wait-early mechanism.
+ *
+ * The status of the previous node will be checked at fixed interval
+ * controlled by PV_PREV_CHECK_MASK. This is to ensure that we won't
+ * pound on the cacheline of the previous node too heavily.
+ */
+#define PV_PREV_CHECK_MASK 0xff
+
+/*
+ * Queue node uses: vcpu_running & vcpu_halted.
+ * Queue head uses: vcpu_running & vcpu_hashed.
+ */
+enum vcpu_state {
+ vcpu_running = 0,
+ vcpu_halted, /* Used only in pv_wait_node */
+ vcpu_hashed, /* = pv_hash'ed + vcpu_halted */
+};
+
+struct pv_node {
+ struct mcs_spinlock mcs;
+ int cpu;
+ u8 state;
+};
+
+/*
+ * Hybrid PV queued/unfair lock
+ *
+ * By replacing the regular queued_spin_trylock() with the function below,
+ * it will be called once when a lock waiter enter the PV slowpath before
+ * being queued.
+ *
+ * The pending bit is set by the queue head vCPU of the MCS wait queue in
+ * pv_wait_head_or_lock() to signal that it is ready to spin on the lock.
+ * When that bit becomes visible to the incoming waiters, no lock stealing
+ * is allowed. The function will return immediately to make the waiters
+ * enter the MCS wait queue. So lock starvation shouldn't happen as long
+ * as the queued mode vCPUs are actively running to set the pending bit
+ * and hence disabling lock stealing.
+ *
+ * When the pending bit isn't set, the lock waiters will stay in the unfair
+ * mode spinning on the lock unless the MCS wait queue is empty. In this
+ * case, the lock waiters will enter the queued mode slowpath trying to
+ * become the queue head and set the pending bit.
+ *
+ * This hybrid PV queued/unfair lock combines the best attributes of a
+ * queued lock (no lock starvation) and an unfair lock (good performance
+ * on not heavily contended locks).
+ */
+#define queued_spin_trylock(l) pv_hybrid_queued_unfair_trylock(l)
+static inline bool pv_hybrid_queued_unfair_trylock(struct qspinlock *lock)
+{
+ /*
+ * Stay in unfair lock mode as long as queued mode waiters are
+ * present in the MCS wait queue but the pending bit isn't set.
+ */
+ for (;;) {
+ int val = atomic_read(&lock->val);
+
+ if (!(val & _Q_LOCKED_PENDING_MASK) &&
+ (cmpxchg_acquire(&lock->locked, 0, _Q_LOCKED_VAL) == 0)) {
+ lockevent_inc(pv_lock_stealing);
+ return true;
+ }
+ if (!(val & _Q_TAIL_MASK) || (val & _Q_PENDING_MASK))
+ break;
+
+ cpu_relax();
+ }
+
+ return false;
+}
+
+/*
+ * The pending bit is used by the queue head vCPU to indicate that it
+ * is actively spinning on the lock and no lock stealing is allowed.
+ */
+#if _Q_PENDING_BITS == 8
+static __always_inline void set_pending(struct qspinlock *lock)
+{
+ WRITE_ONCE(lock->pending, 1);
+}
+
+/*
+ * The pending bit check in pv_queued_spin_steal_lock() isn't a memory
+ * barrier. Therefore, an atomic cmpxchg_acquire() is used to acquire the
+ * lock just to be sure that it will get it.
+ */
+static __always_inline int trylock_clear_pending(struct qspinlock *lock)
+{
+ return !READ_ONCE(lock->locked) &&
+ (cmpxchg_acquire(&lock->locked_pending, _Q_PENDING_VAL,
+ _Q_LOCKED_VAL) == _Q_PENDING_VAL);
+}
+#else /* _Q_PENDING_BITS == 8 */
+static __always_inline void set_pending(struct qspinlock *lock)
+{
+ atomic_or(_Q_PENDING_VAL, &lock->val);
+}
+
+static __always_inline int trylock_clear_pending(struct qspinlock *lock)
+{
+ int val = atomic_read(&lock->val);
+
+ for (;;) {
+ int old, new;
+
+ if (val & _Q_LOCKED_MASK)
+ break;
+
+ /*
+ * Try to clear pending bit & set locked bit
+ */
+ old = val;
+ new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
+ val = atomic_cmpxchg_acquire(&lock->val, old, new);
+
+ if (val == old)
+ return 1;
+ }
+ return 0;
+}
+#endif /* _Q_PENDING_BITS == 8 */
+
+/*
+ * Lock and MCS node addresses hash table for fast lookup
+ *
+ * Hashing is done on a per-cacheline basis to minimize the need to access
+ * more than one cacheline.
+ *
+ * Dynamically allocate a hash table big enough to hold at least 4X the
+ * number of possible cpus in the system. Allocation is done on page
+ * granularity. So the minimum number of hash buckets should be at least
+ * 256 (64-bit) or 512 (32-bit) to fully utilize a 4k page.
+ *
+ * Since we should not be holding locks from NMI context (very rare indeed) the
+ * max load factor is 0.75, which is around the point where open addressing
+ * breaks down.
+ *
+ */
+struct pv_hash_entry {
+ struct qspinlock *lock;
+ struct pv_node *node;
+};
+
+#define PV_HE_PER_LINE (SMP_CACHE_BYTES / sizeof(struct pv_hash_entry))
+#define PV_HE_MIN (PAGE_SIZE / sizeof(struct pv_hash_entry))
+
+static struct pv_hash_entry *pv_lock_hash;
+static unsigned int pv_lock_hash_bits __read_mostly;
+
+/*
+ * Allocate memory for the PV qspinlock hash buckets
+ *
+ * This function should be called from the paravirt spinlock initialization
+ * routine.
+ */
+void __init __pv_init_lock_hash(void)
+{
+ int pv_hash_size = ALIGN(4 * num_possible_cpus(), PV_HE_PER_LINE);
+
+ if (pv_hash_size < PV_HE_MIN)
+ pv_hash_size = PV_HE_MIN;
+
+ /*
+ * Allocate space from bootmem which should be page-size aligned
+ * and hence cacheline aligned.
+ */
+ pv_lock_hash = alloc_large_system_hash("PV qspinlock",
+ sizeof(struct pv_hash_entry),
+ pv_hash_size, 0,
+ HASH_EARLY | HASH_ZERO,
+ &pv_lock_hash_bits, NULL,
+ pv_hash_size, pv_hash_size);
+}
+
+#define for_each_hash_entry(he, offset, hash) \
+ for (hash &= ~(PV_HE_PER_LINE - 1), he = &pv_lock_hash[hash], offset = 0; \
+ offset < (1 << pv_lock_hash_bits); \
+ offset++, he = &pv_lock_hash[(hash + offset) & ((1 << pv_lock_hash_bits) - 1)])
+
+static struct qspinlock **pv_hash(struct qspinlock *lock, struct pv_node *node)
+{
+ unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
+ struct pv_hash_entry *he;
+ int hopcnt = 0;
+
+ for_each_hash_entry(he, offset, hash) {
+ hopcnt++;
+ if (!cmpxchg(&he->lock, NULL, lock)) {
+ WRITE_ONCE(he->node, node);
+ lockevent_pv_hop(hopcnt);
+ return &he->lock;
+ }
+ }
+ /*
+ * Hard assume there is a free entry for us.
+ *
+ * This is guaranteed by ensuring every blocked lock only ever consumes
+ * a single entry, and since we only have 4 nesting levels per CPU
+ * and allocated 4*nr_possible_cpus(), this must be so.
+ *
+ * The single entry is guaranteed by having the lock owner unhash
+ * before it releases.
+ */
+ BUG();
+}
+
+static struct pv_node *pv_unhash(struct qspinlock *lock)
+{
+ unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
+ struct pv_hash_entry *he;
+ struct pv_node *node;
+
+ for_each_hash_entry(he, offset, hash) {
+ if (READ_ONCE(he->lock) == lock) {
+ node = READ_ONCE(he->node);
+ WRITE_ONCE(he->lock, NULL);
+ return node;
+ }
+ }
+ /*
+ * Hard assume we'll find an entry.
+ *
+ * This guarantees a limited lookup time and is itself guaranteed by
+ * having the lock owner do the unhash -- IFF the unlock sees the
+ * SLOW flag, there MUST be a hash entry.
+ */
+ BUG();
+}
+
+/*
+ * Return true if when it is time to check the previous node which is not
+ * in a running state.
+ */
+static inline bool
+pv_wait_early(struct pv_node *prev, int loop)
+{
+ if ((loop & PV_PREV_CHECK_MASK) != 0)
+ return false;
+
+ return READ_ONCE(prev->state) != vcpu_running;
+}
+
+/*
+ * 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) > sizeof(struct qnode));
+
+ 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 set _Q_SLOW_VAL and fill in hash table on its
+ * behalf.
+ */
+static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
+{
+ struct pv_node *pn = (struct pv_node *)node;
+ struct pv_node *pp = (struct pv_node *)prev;
+ int loop;
+ bool wait_early;
+
+ for (;;) {
+ for (wait_early = false, loop = SPIN_THRESHOLD; loop; loop--) {
+ if (READ_ONCE(node->locked))
+ return;
+ if (pv_wait_early(pp, loop)) {
+ wait_early = true;
+ break;
+ }
+ 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_hashed
+ *
+ * Matches the cmpxchg() from pv_kick_node().
+ */
+ smp_store_mb(pn->state, vcpu_halted);
+
+ if (!READ_ONCE(node->locked)) {
+ lockevent_inc(pv_wait_node);
+ lockevent_cond_inc(pv_wait_early, wait_early);
+ pv_wait(&pn->state, vcpu_halted);
+ }
+
+ /*
+ * If pv_kick_node() changed us to vcpu_hashed, retain that
+ * value so that pv_wait_head_or_lock() knows to not also try
+ * to hash this lock.
+ */
+ cmpxchg(&pn->state, vcpu_halted, vcpu_running);
+
+ /*
+ * If the locked flag is still not set after wakeup, it is a
+ * spurious wakeup and the vCPU should wait again. However,
+ * there is a pretty high overhead for CPU halting and kicking.
+ * So it is better to spin for a while in the hope that the
+ * MCS lock will be released soon.
+ */
+ lockevent_cond_inc(pv_spurious_wakeup,
+ !READ_ONCE(node->locked));
+ }
+
+ /*
+ * 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 when we're the lock owner.
+ *
+ * Instead of waking the waiters stuck in pv_wait_node() advance their state
+ * such that they're waiting in pv_wait_head_or_lock(), this avoids a
+ * wake/sleep cycle.
+ */
+static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
+{
+ struct pv_node *pn = (struct pv_node *)node;
+
+ /*
+ * If the vCPU is indeed halted, advance its state to match that of
+ * pv_wait_node(). If OTOH this fails, the vCPU was running and will
+ * observe its next->locked value and advance itself.
+ *
+ * Matches with smp_store_mb() and cmpxchg() in pv_wait_node()
+ *
+ * The write to next->locked in arch_mcs_spin_unlock_contended()
+ * must be ordered before the read of pn->state in the cmpxchg()
+ * below for the code to work correctly. To guarantee full ordering
+ * irrespective of the success or failure of the cmpxchg(),
+ * a relaxed version with explicit barrier is used. The control
+ * dependency will order the reading of pn->state before any
+ * subsequent writes.
+ */
+ smp_mb__before_atomic();
+ if (cmpxchg_relaxed(&pn->state, vcpu_halted, vcpu_hashed)
+ != vcpu_halted)
+ return;
+
+ /*
+ * Put the lock into the hash table and set the _Q_SLOW_VAL.
+ *
+ * As this is the same vCPU that will check the _Q_SLOW_VAL value and
+ * the hash table later on at unlock time, no atomic instruction is
+ * needed.
+ */
+ WRITE_ONCE(lock->locked, _Q_SLOW_VAL);
+ (void)pv_hash(lock, pn);
+}
+
+/*
+ * Wait for l->locked to become clear and acquire the lock;
+ * halt the vcpu after a short spin.
+ * __pv_queued_spin_unlock() will wake us.
+ *
+ * The current value of the lock will be returned for additional processing.
+ */
+static u32
+pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
+{
+ struct pv_node *pn = (struct pv_node *)node;
+ struct qspinlock **lp = NULL;
+ int waitcnt = 0;
+ int loop;
+
+ /*
+ * If pv_kick_node() already advanced our state, we don't need to
+ * insert ourselves into the hash table anymore.
+ */
+ if (READ_ONCE(pn->state) == vcpu_hashed)
+ lp = (struct qspinlock **)1;
+
+ /*
+ * Tracking # of slowpath locking operations
+ */
+ lockevent_inc(lock_slowpath);
+
+ for (;; waitcnt++) {
+ /*
+ * Set correct vCPU state to be used by queue node wait-early
+ * mechanism.
+ */
+ WRITE_ONCE(pn->state, vcpu_running);
+
+ /*
+ * Set the pending bit in the active lock spinning loop to
+ * disable lock stealing before attempting to acquire the lock.
+ */
+ set_pending(lock);
+ for (loop = SPIN_THRESHOLD; loop; loop--) {
+ if (trylock_clear_pending(lock))
+ goto gotlock;
+ cpu_relax();
+ }
+ clear_pending(lock);
+
+
+ if (!lp) { /* ONCE */
+ lp = pv_hash(lock, pn);
+
+ /*
+ * We must hash before setting _Q_SLOW_VAL, such that
+ * when we observe _Q_SLOW_VAL in __pv_queued_spin_unlock()
+ * we'll be sure to be able to observe our hash entry.
+ *
+ * [S] <hash> [Rmw] l->locked == _Q_SLOW_VAL
+ * MB RMB
+ * [RmW] l->locked = _Q_SLOW_VAL [L] <unhash>
+ *
+ * Matches the smp_rmb() in __pv_queued_spin_unlock().
+ */
+ if (xchg(&lock->locked, _Q_SLOW_VAL) == 0) {
+ /*
+ * The lock was free and now we own the lock.
+ * Change the lock value back to _Q_LOCKED_VAL
+ * and unhash the table.
+ */
+ WRITE_ONCE(lock->locked, _Q_LOCKED_VAL);
+ WRITE_ONCE(*lp, NULL);
+ goto gotlock;
+ }
+ }
+ WRITE_ONCE(pn->state, vcpu_hashed);
+ lockevent_inc(pv_wait_head);
+ lockevent_cond_inc(pv_wait_again, waitcnt);
+ pv_wait(&lock->locked, _Q_SLOW_VAL);
+
+ /*
+ * Because of lock stealing, the queue head vCPU may not be
+ * able to acquire the lock before it has to wait again.
+ */
+ }
+
+ /*
+ * The cmpxchg() or xchg() call before coming here provides the
+ * acquire semantics for locking. The dummy ORing of _Q_LOCKED_VAL
+ * here is to indicate to the compiler that the value will always
+ * be nozero to enable better code optimization.
+ */
+gotlock:
+ return (u32)(atomic_read(&lock->val) | _Q_LOCKED_VAL);
+}
+
+/*
+ * PV versions of the unlock fastpath and slowpath functions to be used
+ * instead of queued_spin_unlock().
+ */
+__visible __lockfunc void
+__pv_queued_spin_unlock_slowpath(struct qspinlock *lock, u8 locked)
+{
+ struct pv_node *node;
+
+ if (unlikely(locked != _Q_SLOW_VAL)) {
+ WARN(!debug_locks_silent,
+ "pvqspinlock: lock 0x%lx has corrupted value 0x%x!\n",
+ (unsigned long)lock, atomic_read(&lock->val));
+ return;
+ }
+
+ /*
+ * A failed cmpxchg doesn't provide any memory-ordering guarantees,
+ * so we need a barrier to order the read of the node data in
+ * pv_unhash *after* we've read the lock being _Q_SLOW_VAL.
+ *
+ * Matches the cmpxchg() in pv_wait_head_or_lock() setting _Q_SLOW_VAL.
+ */
+ smp_rmb();
+
+ /*
+ * Since the above failed to release, this must be the SLOW path.
+ * Therefore start by looking up the blocked node and unhashing it.
+ */
+ node = pv_unhash(lock);
+
+ /*
+ * Now that we have a reference to the (likely) blocked pv_node,
+ * release the lock.
+ */
+ smp_store_release(&lock->locked, 0);
+
+ /*
+ * At this point the memory pointed at by lock can be freed/reused,
+ * however we can still use the pv_node to kick the CPU.
+ * The other vCPU may not really be halted, but kicking an active
+ * vCPU is harmless other than the additional latency in completing
+ * the unlock.
+ */
+ lockevent_inc(pv_kick_unlock);
+ pv_kick(node->cpu);
+}
+
+/*
+ * Include the architecture specific callee-save thunk of the
+ * __pv_queued_spin_unlock(). This thunk is put together with
+ * __pv_queued_spin_unlock() to make the callee-save thunk and the real unlock
+ * function close to each other sharing consecutive instruction cachelines.
+ * Alternatively, architecture specific version of __pv_queued_spin_unlock()
+ * can be defined.
+ */
+#include <asm/qspinlock_paravirt.h>
+
+#ifndef __pv_queued_spin_unlock
+__visible __lockfunc void __pv_queued_spin_unlock(struct qspinlock *lock)
+{
+ u8 locked;
+
+ /*
+ * We must not unlock if SLOW, because in that case we must first
+ * unhash. Otherwise it would be possible to have multiple @lock
+ * entries, which would be BAD.
+ */
+ locked = cmpxchg_release(&lock->locked, _Q_LOCKED_VAL, 0);
+ if (likely(locked == _Q_LOCKED_VAL))
+ return;
+
+ __pv_queued_spin_unlock_slowpath(lock, locked);
+}
+#endif /* __pv_queued_spin_unlock */
generated by cgit v1.2.3 (git 2.39.1) at 2024-11-24 18:27:02 +0000