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Date: Thu, 2 Apr 2015 21:48:34 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Waiman Long <waiman.long@...com>
Cc: tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
paolo.bonzini@...il.com, konrad.wilk@...cle.com,
boris.ostrovsky@...cle.com, paulmck@...ux.vnet.ibm.com,
riel@...hat.com, torvalds@...ux-foundation.org,
raghavendra.kt@...ux.vnet.ibm.com, david.vrabel@...rix.com,
oleg@...hat.com, scott.norton@...com, doug.hatch@...com,
linux-arch@...r.kernel.org, x86@...nel.org,
linux-kernel@...r.kernel.org,
virtualization@...ts.linux-foundation.org,
xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
luto@...capital.net
Subject: Re: [PATCH 8/9] qspinlock: Generic paravirt support
On Thu, Apr 02, 2015 at 07:20:57PM +0200, Peter Zijlstra wrote:
> pv_wait_head():
>
> pv_hash()
> /* MB as per cmpxchg */
> cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL);
>
> VS
>
> __pv_queue_spin_unlock():
>
> if (xchg(&l->locked, 0) != _Q_SLOW_VAL)
> return;
>
> /* MB as per xchg */
> pv_hash_find(lock);
>
>
Something like so.. compile tested only.
I took out the LFSR because that was likely over engineering from my
side :-)
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -2,6 +2,8 @@
#error "do not include this file"
#endif
+#include <linux/hash.h>
+
/*
* Implement paravirt qspinlocks; the general idea is to halt the vcpus instead
* of spinning them.
@@ -107,7 +109,84 @@ static void pv_kick_node(struct mcs_spin
pv_kick(pn->cpu);
}
-static DEFINE_PER_CPU(struct qspinlock *, __pv_lock_wait);
+/*
+ * Hash table using open addressing with an linear probe sequence.
+ *
+ * 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 adressing
+ * breaks down.
+ *
+ * Instead of probing just the immediate bucket we probe all buckets in the
+ * same cacheline.
+ *
+ * http://en.wikipedia.org/wiki/Hash_table#Open_addressing
+ *
+ */
+
+struct pv_hash_bucket {
+ struct qspinlock *lock;
+ int cpu;
+};
+
+/*
+ * XXX dynamic allocate using nr_cpu_ids instead...
+ */
+#define PV_LOCK_HASH_BITS (2 + NR_CPUS_BITS)
+
+#if PV_LOCK_HASH_BITS < 6
+#undef PV_LOCK_HASH_BITS
+#define PB_LOCK_HASH_BITS 6
+#endif
+
+#define PV_LOCK_HASH_SIZE (1 << PV_LOCK_HASH_BITS)
+
+static struct pv_hash_bucket __pv_lock_hash[PV_LOCK_HASH_SIZE] ____cacheline_aligned;
+
+#define PV_HB_PER_LINE (SMP_CACHE_BYTES / sizeof(struct pv_hash_bucket))
+
+static inline u32 hash_align(u32 hash)
+{
+ return hash & ~(PV_HB_PER_LINE - 1);
+}
+
+#define for_each_hash_bucket(hb, off, hash) \
+ for (hash = hash_align(hash), off = 0, hb = &__pv_lock_hash[hash + off];\
+ off < PV_LOCK_HASH_SIZE; \
+ off++, hb = &__pv_lock_hash[(hash + off) % PV_LOCK_HASH_SIZE])
+
+static struct pv_hash_bucket *pv_hash_insert(struct qspinlock *lock)
+{
+ u32 offset, hash = hash_ptr(lock, PV_LOCK_HASH_BITS);
+ struct pv_hash_bucket *hb;
+
+ for_each_hash_bucket(hb, offset, hash) {
+ if (!cmpxchg(&hb->lock, NULL, lock)) {
+ WRITE_ONCE(hb->cpu, smp_processor_id());
+ return hb;
+ }
+ }
+
+ /*
+ * Hard assumes there is an empty bucket somewhere.
+ */
+ BUG();
+}
+
+static struct pv_hash_bucket *pv_hash_find(struct qspinlock *lock)
+{
+ u32 offset, hash = hash_ptr(lock, PV_LOCK_HASH_BITS);
+ struct pv_hash_bucket *hb;
+
+ for_each_hash_bucket(hb, offset, hash) {
+ if (READ_ONCE(hb->lock) == lock)
+ return hb;
+ }
+
+ /*
+ * Hard assumes we _WILL_ find a match.
+ */
+ BUG();
+}
/*
* Wait for l->locked to become clear; halt the vcpu after a short spin.
@@ -116,7 +195,9 @@ static DEFINE_PER_CPU(struct qspinlock *
static void pv_wait_head(struct qspinlock *lock)
{
struct __qspinlock *l = (void *)lock;
+ struct pv_hash_bucket *hb = NULL;
int loop;
+ u8 o;
for (;;) {
for (loop = SPIN_THRESHOLD; loop; loop--) {
@@ -126,29 +207,47 @@ static void pv_wait_head(struct qspinloc
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;
+ if (!hb) {
+ hb = pv_hash_insert(lock);
+ /*
+ * hb must be set before setting _Q_SLOW_VAL
+ *
+ * [S] hb <- lock [RmW] l = l->locked = 0
+ * MB MB
+ * [RmW] l->locked ?= _Q_SLOW_VAL [L] hb
+ *
+ * Matches the xchg() in pv_queue_spin_unlock().
+ */
+ o = cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL);
+ if (!o) {
+ /*
+ * The lock got unlocked before we could set
+ * _Q_SLOW_VAL, we must unhash ourselves.
+ */
+ WRITE_ONCE(hb->lock, NULL);
+ goto done;
+ }
+ BUG_ON(o != _Q_LOCKED_VAL);
+ /*
+ * At this point _Q_SLOW_VAL is visible and the unlock
+ * will do the lookup. The lookup hard relies on the
+ * entry being visible -- which it should be. Unlock
+ * will unhash for us.
+ */
+ }
pv_wait(&l->locked, _Q_SLOW_VAL);
+ /*
+ * We can get spurious wakeups from interrupts, cycle back.
+ */
}
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.
*/
+ return;
}
/*
@@ -158,20 +257,20 @@ static void pv_wait_head(struct qspinloc
void __pv_queue_spin_unlock(struct qspinlock *lock)
{
struct __qspinlock *l = (void *)lock;
- int cpu;
+ struct pv_hash_bucket *hb;
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.
+ * however we can still use the pointer value to search in our hash
+ * table.
*
- * XXX: get rid of this loop
+ * Also, if we observe _Q_SLOW_VAL we _must_ now observe 'our' hash
+ * bucket. See pv_wait_head().
*/
- for_each_possible_cpu(cpu) {
- if (per_cpu(__pv_lock_wait, cpu) == lock)
- pv_kick(cpu);
- }
+ hb = pv_hash_find(lock);
+ pv_kick(hb->cpu);
+ WRITE_ONCE(hb->lock, NULL); /* unhash */
}
--
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