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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-07 18:49:45 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-07 18:49:45 +0000 |
commit | 2c3c1048746a4622d8c89a29670120dc8fab93c4 (patch) | |
tree | 848558de17fb3008cdf4d861b01ac7781903ce39 /kernel/locking/qspinlock_paravirt.h | |
parent | Initial commit. (diff) | |
download | linux-2c3c1048746a4622d8c89a29670120dc8fab93c4.tar.xz linux-2c3c1048746a4622d8c89a29670120dc8fab93c4.zip |
Adding upstream version 6.1.76.upstream/6.1.76
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'kernel/locking/qspinlock_paravirt.h')
-rw-r--r-- | kernel/locking/qspinlock_paravirt.h | 562 |
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 */ |