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authorWaiman Long <Waiman.Long@hp.com>2015-04-24 20:56:37 +0200
committerIngo Molnar <mingo@kernel.org>2015-05-08 12:37:05 +0200
commita23db284fe0d1879ca2002bf31077b5efa2fe2ca (patch)
tree816042635ecb0462f1cb3f35bc8091f1cd46ce19 /kernel/locking/qspinlock_paravirt.h
parentlocking/qspinlock: Revert to test-and-set on hypervisors (diff)
downloadlinux-a23db284fe0d1879ca2002bf31077b5efa2fe2ca.tar.xz
linux-a23db284fe0d1879ca2002bf31077b5efa2fe2ca.zip
locking/pvqspinlock: 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 directly visible to the unlocker. So the unlocker searches a hash table with open addressing using a simple binary Galois linear feedback shift register. Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Waiman Long <Waiman.Long@hp.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> Cc: Borislav Petkov <bp@alien8.de> Cc: Daniel J Blueman <daniel@numascale.com> Cc: David Vrabel <david.vrabel@citrix.com> Cc: Douglas Hatch <doug.hatch@hp.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Paolo Bonzini <paolo.bonzini@gmail.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> Cc: Rik van Riel <riel@redhat.com> Cc: Scott J Norton <scott.norton@hp.com> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/1429901803-29771-9-git-send-email-Waiman.Long@hp.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/locking/qspinlock_paravirt.h')
-rw-r--r--kernel/locking/qspinlock_paravirt.h325
1 files changed, 325 insertions, 0 deletions
diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
new file mode 100644
index 000000000000..b5758a95a8d3
--- /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_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)
+
+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.
+ *
+ */
+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,
+ &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_queued_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_queued_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
+ * queued_spin_unlock().
+ */
+__visible void __pv_queued_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_queued_spin_unlock(). This thunk is put together with
+ * __pv_queued_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>
+