[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [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 */
 }

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel


 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.