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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v16 08/14] pvqspinlock: Implement simple paravirt support for the qspinlock
I changed it to the below; I've not gotten around to compiling or even running it yet :-( The biggest change is the pv_hash/pv_unhash functions, which I've rewritten to hopefully be clearer (and also hopefully not wrecked them). I took out the cacheline sized structure which takes out that double loop and simplifies things. I've also added some comments which hopefully explain how/why we ended up with this exact scheme. I've also moved the __pv_queue_spin_unlock() function to the tail, such that we keep the 'wait'/'kick' order for both node and head. In any case, like I just wrote on the other email, I've stuck some things in my queue (up to and including patch 11) and if it all works out we can continue from there. --- Subject: pvqspinlock: Implement simple paravirt support for the qspinlock From: Waiman Long <Waiman.Long@xxxxxx> Date: Fri, 24 Apr 2015 14:56:37 -0400 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 directly visible to the unlocker. So the unlocker searches a hash table with open addressing using a simple binary Galois linear feedback shift register. Cc: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx> Cc: David Vrabel <david.vrabel@xxxxxxxxxx> Cc: Oleg Nesterov <oleg@xxxxxxxxxx> Cc: Daniel J Blueman <daniel@xxxxxxxxxxxxx> Cc: Paolo Bonzini <paolo.bonzini@xxxxxxxxx> Cc: Scott J Norton <scott.norton@xxxxxx> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> Cc: Douglas Hatch <doug.hatch@xxxxxx> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Signed-off-by: Waiman Long <Waiman.Long@xxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Link: http://lkml.kernel.org/r/1429901803-29771-9-git-send-email-Waiman.Long@xxxxxx --- kernel/locking/qspinlock.c | 68 +++++++ kernel/locking/qspinlock_paravirt.h | 325 ++++++++++++++++++++++++++++++++++++ 2 files changed, 392 insertions(+), 1 deletion(-) create mode 100644 kernel/locking/qspinlock_paravirt.h --- 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, @@ -220,6 +231,32 @@ 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, + struct mcs_spinlock *node) { } + +#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 @@ -249,6 +286,9 @@ 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; @@ -325,6 +365,7 @@ 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; @@ -350,6 +391,7 @@ 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); } @@ -365,6 +407,7 @@ void queue_spin_lock_slowpath(struct qsp * does not imply a full barrier. * */ + pv_wait_head(lock, node); while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK) cpu_relax(); @@ -397,6 +440,7 @@ void queue_spin_lock_slowpath(struct qsp cpu_relax(); arch_mcs_spin_unlock_contended(&next->locked); + pv_kick_node(next); release: /* @@ -405,3 +449,25 @@ 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 +1,325 @@ +#ifndef _GEN_PV_LOCK_SLOWPATH +#error "do not include this file" +#endif + +#include <linux/hash.h> +#include <linux/bootmem.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_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 (3U << _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; +}; + +/* + * 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. + * + */ +#define PV_HE_PER_LINE (SMP_CACHE_BYTES / sizeof(struct pv_hash_entry)) +#define PV_HE_MIN (PAGE_SIZE / sizeof(struct pv_hash_entry)) + +struct pv_hash_entry { + struct qspinlock *lock; + struct pv_node *node; +}; + +static struct pv_hash_bucket *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, + &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; + + for_each_hash_entry(he, offset, hash) { + if (!cmpxchg(&he->lock, NULL, lock)) { + WRITE_ONCE(he->node, node); + 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(); +} + +/* + * 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)) + return; + 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)) + pv_wait(&pn->state, vcpu_halted); + + /* + * Reset the vCPU state to avoid unncessary CPU kicking + */ + WRITE_ONCE(pn->state, 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. + */ + } + /* + * 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); +} + +/* + * 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 mcs_spinlock *node) +{ + struct pv_node *pn = (struct pv_node *)node; + struct __qspinlock *l = (void *)lock; + struct qspinlock **lp = NULL; + int loop; + + for (;;) { + for (loop = SPIN_THRESHOLD; loop; loop--) { + if (!READ_ONCE(l->locked)) + return; + cpu_relax(); + } + + WRITE_ONCE(pn->state, vcpu_halted); + if (!lp) { /* ONCE */ + lp = pv_hash(lock, pn); + /* + * lp must be set before setting _Q_SLOW_VAL + * + * [S] lp = lock [RmW] l = l->locked = 0 + * MB MB + * [S] l->locked = _Q_SLOW_VAL [L] lp + * + * Matches the cmpxchg() in __pv_queue_spin_unlock(). + */ + if (!cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL)) { + /* + * The lock is free and _Q_SLOW_VAL has never + * been set. Therefore we need to unhash before + * getting the lock. + */ + WRITE_ONCE(*lp, NULL); + return; + } + } + pv_wait(&l->locked, _Q_SLOW_VAL); + + /* + * The unlocker should have freed the lock before kicking the + * CPU. So if the lock is still not free, it is a spurious + * wakeup and so the vCPU should wait again after spinning for + * a while. + */ + } + + /* + * 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. + */ +} + +/* + * PV version of the unlock function to be used in stead of + * queue_spin_unlock(). + */ +__visible void __pv_queue_spin_unlock(struct qspinlock *lock) +{ + struct __qspinlock *l = (void *)lock; + struct pv_node *node; + + /* + * 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. + */ + if (likely(cmpxchg(&l->locked, _Q_LOCKED_VAL, 0) == _Q_LOCKED_VAL)) + return; + + /* + * 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(&l->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. + */ + if (READ_ONCE(node->state) == vcpu_halted) + pv_kick(node->cpu); +} +/* + * Include the architecture specific callee-save thunk of the + * __pv_queue_spin_unlock(). This thunk is put together with + * __pv_queue_spin_unlock() near the top of the file to make sure + * that the callee-save thunk and the real unlock function are close + * to each other sharing consecutive instruction cachelines. + */ +#include <asm/qspinlock_paravirt.h> + _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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