2 files changed, 15 insertions, 8 deletions

diff --git a/fs/xfs/support/ktrace.c b/fs/xfs/support/ktrace.c
index 4e0444c0aca..0b75d302508 100644
--- a/fs/xfs/support/ktrace.c
+++ b/fs/xfs/support/ktrace.c
@@ -24,7 +24,7 @@ static int          ktrace_zentries;
 void __init
 ktrace_init(int zentries)
 {
-	ktrace_zentries = zentries;
+	ktrace_zentries = roundup_pow_of_two(zentries);
 
 	ktrace_hdr_zone = kmem_zone_init(sizeof(ktrace_t),
 					"ktrace_hdr");
@@ -47,13 +47,16 @@ ktrace_uninit(void)
  * ktrace_alloc()
  *
  * Allocate a ktrace header and enough buffering for the given
- * number of entries.
+ * number of entries. Round the number of entries up to a
+ * power of 2 so we can do fast masking to get the index from
+ * the atomic index counter.
  */
 ktrace_t *
 ktrace_alloc(int nentries, unsigned int __nocast sleep)
 {
 	ktrace_t        *ktp;
 	ktrace_entry_t  *ktep;
+	int		entries;
 
 	ktp = (ktrace_t*)kmem_zone_alloc(ktrace_hdr_zone, sleep);
 
@@ -70,11 +73,12 @@ ktrace_alloc(int nentries, unsigned int __nocast sleep)
 	/*
 	 * Special treatment for buffers with the ktrace_zentries entries
 	 */
-	if (nentries == ktrace_zentries) {
+	entries = roundup_pow_of_two(nentries);
+	if (entries == ktrace_zentries) {
 		ktep = (ktrace_entry_t*)kmem_zone_zalloc(ktrace_ent_zone,
 							    sleep);
 	} else {
-		ktep = (ktrace_entry_t*)kmem_zalloc((nentries * sizeof(*ktep)),
+		ktep = (ktrace_entry_t*)kmem_zalloc((entries * sizeof(*ktep)),
 							    sleep | KM_LARGE);
 	}
 
@@ -91,7 +95,9 @@ ktrace_alloc(int nentries, unsigned int __nocast sleep)
 	}
 
 	ktp->kt_entries  = ktep;
-	ktp->kt_nentries = nentries;
+	ktp->kt_nentries = entries;
+	ASSERT(is_power_of_2(entries));
+	ktp->kt_index_mask = entries - 1;
 	atomic_set(&ktp->kt_index, 0);
 	ktp->kt_rollover = 0;
 	return ktp;
@@ -160,7 +166,7 @@ ktrace_enter(
 	 * Grab an entry by pushing the index up to the next one.
 	 */
 	index = atomic_add_return(1, &ktp->kt_index);
-	index = (index - 1) % ktp->kt_nentries;
+	index = (index - 1) & ktp->kt_index_mask;
 	if (!ktp->kt_rollover && index == ktp->kt_nentries - 1)
 		ktp->kt_rollover = 1;
 
@@ -197,7 +203,7 @@ ktrace_nentries(
 	if (ktp == NULL)
 		return 0;
 
-	index = atomic_read(&ktp->kt_index) % ktp->kt_nentries;
+	index = atomic_read(&ktp->kt_index) & ktp->kt_index_mask;
 	return (ktp->kt_rollover ? ktp->kt_nentries : index);
 }
 
@@ -223,7 +229,7 @@ ktrace_first(ktrace_t   *ktp, ktrace_snap_t     *ktsp)
 	int             nentries;
 
 	if (ktp->kt_rollover)
-		index = atomic_read(&ktp->kt_index) % ktp->kt_nentries;
+		index = atomic_read(&ktp->kt_index) & ktp->kt_index_mask;
 	else
 		index = 0;
 
diff --git a/fs/xfs/support/ktrace.h b/fs/xfs/support/ktrace.h
index 782dbbb6a9d..741d6947ca6 100644
--- a/fs/xfs/support/ktrace.h
+++ b/fs/xfs/support/ktrace.h
@@ -31,6 +31,7 @@ typedef struct ktrace_entry {
 typedef struct ktrace {
 	int		kt_nentries;	/* number of entries in trace buf */
 	atomic_t	kt_index;	/* current index in entries */
+	unsigned int	kt_index_mask;
 	int		kt_rollover;
 	ktrace_entry_t	*kt_entries;	/* buffer of entries */
 } ktrace_t;