Re: [Qemu-devel] [RFC PATCH] qht: Align sequence lock to cache line

[Pranith Kumar]

Paolo Bonzini writes:

> On 25/10/2016 17:49, Pranith Kumar wrote:
>> But we are taking the seqlock of only the head bucket, while the
>> readers are reading hashes/pointers of the chained buckets.
>
> No, we aren't.  See qht_lookup__slowpath.


I don't see it. The reader is taking the head bucket look in
qht_lookup__slowpath() and then iterating over the chained buckets in
qht_do_lookup(). The writer is doing the same. It is taking the head bucket
lock in qht_insert__locked().

I've written a patch (see below) to take the per-bucket sequence locks.

>
> This patch:
>
> throughput base   patch  %change
> update
> 0          8.07   13.33  +65%
> 10         7.10   8.90   +25%
> 20         6.34   7.02   +10%
> 30         5.48   6.11   +9.6%
> 40         4.90   5.46   +11.42%
>
>
> Just doubling the cachesize:
>
> throughput base   patch  %change
> update
> 0          8.07   4.47   -45% ?!?
> 10         7.10   9.82   +38%
> 20         6.34   8.13   +28%
> 30         5.48   7.13   +30%
> 40         5.90   6.45   +30%
>
> It seems to me that your machine has 128-byte cachelines.
>

Nope. It is just the regular 64 byte cache line.

$ getconf LEVEL1_DCACHE_LINESIZE
64

(The machine model is Xeon CPU E5-2620).


Take the per-bucket sequence locks instead of the head bucket lock.

Signed-off-by: Pranith Kumar <address@hidden>
---
 util/qht.c | 36 ++++++++++++++++++------------------
 1 file changed, 18 insertions(+), 18 deletions(-)

diff --git a/util/qht.c b/util/qht.c
index 4d82609..cfce5fc 100644
--- a/util/qht.c
+++ b/util/qht.c
@@ -374,19 +374,19 @@ static void qht_bucket_reset__locked(struct qht_bucket 
*head)
     struct qht_bucket *b = head;
     int i;
 
-    seqlock_write_begin(&head->sequence);
     do {
+        seqlock_write_begin(&b->sequence);
         for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
             if (b->pointers[i] == NULL) {
-                goto done;
+                seqlock_write_end(&b->sequence);
+                return;
             }
             atomic_set(&b->hashes[i], 0);
             atomic_set(&b->pointers[i], NULL);
         }
+        seqlock_write_end(&b->sequence);
         b = b->next;
     } while (b);
- done:
-    seqlock_write_end(&head->sequence);
 }
 
 /* call with all bucket locks held */
@@ -446,6 +446,8 @@ void *qht_do_lookup(struct qht_bucket *head, 
qht_lookup_func_t func,
     int i;
 
     do {
+        void *q = NULL;
+        unsigned int version = seqlock_read_begin(&b->sequence);
         for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
             if (atomic_read(&b->hashes[i]) == hash) {
                 /* The pointer is dereferenced before seqlock_read_retry,
@@ -455,11 +457,16 @@ void *qht_do_lookup(struct qht_bucket *head, 
qht_lookup_func_t func,
                 void *p = atomic_rcu_read(&b->pointers[i]);
 
                 if (likely(p) && likely(func(p, userp))) {
-                    return p;
+                    q = p;
+                    break;
                 }
             }
         }
-        b = atomic_rcu_read(&b->next);
+        if (!q) {
+            b = atomic_rcu_read(&b->next);
+        } else if (!seqlock_read_retry(&b->sequence, version)) {
+            return q;
+        }
     } while (b);
 
     return NULL;
@@ -469,14 +476,7 @@ static __attribute__((noinline))
 void *qht_lookup__slowpath(struct qht_bucket *b, qht_lookup_func_t func,
                            const void *userp, uint32_t hash)
 {
-    unsigned int version;
-    void *ret;
-
-    do {
-        version = seqlock_read_begin(&b->sequence);
-        ret = qht_do_lookup(b, func, userp, hash);
-    } while (seqlock_read_retry(&b->sequence, version));
-    return ret;
+    return qht_do_lookup(b, func, userp, hash);
 }
 
 void *qht_lookup(struct qht *ht, qht_lookup_func_t func, const void *userp,
@@ -537,14 +537,14 @@ static bool qht_insert__locked(struct qht *ht, struct 
qht_map *map,
 
  found:
     /* found an empty key: acquire the seqlock and write */
-    seqlock_write_begin(&head->sequence);
+    seqlock_write_begin(&b->sequence);
     if (new) {
         atomic_rcu_set(&prev->next, b);
     }
     /* smp_wmb() implicit in seqlock_write_begin.  */
     atomic_set(&b->hashes[i], hash);
     atomic_set(&b->pointers[i], p);
-    seqlock_write_end(&head->sequence);
+    seqlock_write_end(&b->sequence);
     return true;
 }
 
@@ -665,9 +665,9 @@ bool qht_remove__locked(struct qht_map *map, struct 
qht_bucket *head,
             }
             if (q == p) {
                 qht_debug_assert(b->hashes[i] == hash);
-                seqlock_write_begin(&head->sequence);
+                seqlock_write_begin(&b->sequence);
                 qht_bucket_remove_entry(b, i);
-                seqlock_write_end(&head->sequence);
+                seqlock_write_end(&b->sequence);
                 return true;
             }
         }
-- 
2.10.1