[patch, debug, 1/2] use atomic counters for ktrace buffer indexes

[David Chinner <dgc@xxxxxxx>]

ktrace_enter() is consuming vast amounts of CPU time
due to the use of a single global lock for protecting
buffer index increments. Change it to use per-buffer
atomic counters - this reduces ktrace_enter() overhead
during a trace intensive test on a 4p machine from 58%
of all CPU time to 12% and halves test runtime.

Signed-off-by: Dave Chinner <dgc@xxxxxxx>
---
 fs/xfs/support/ktrace.c |   21 ++++++++-------------
 fs/xfs/support/ktrace.h |    2 +-
 2 files changed, 9 insertions(+), 14 deletions(-)

Index: 2.6.x-xfs-new/fs/xfs/support/ktrace.c
===================================================================
--- 2.6.x-xfs-new.orig/fs/xfs/support/ktrace.c  2008-02-15 17:02:00.000000000 
+1100
+++ 2.6.x-xfs-new/fs/xfs/support/ktrace.c       2008-02-18 15:33:13.849314747 
+1100
@@ -92,7 +92,7 @@ ktrace_alloc(int nentries, unsigned int 
 
        ktp->kt_entries  = ktep;
        ktp->kt_nentries = nentries;
-       ktp->kt_index    = 0;
+       atomic_set(&ktp->kt_index, 0);
        ktp->kt_rollover = 0;
        return ktp;
 }
@@ -151,8 +151,6 @@ ktrace_enter(
        void            *val14,
        void            *val15)
 {
-       static DEFINE_SPINLOCK(wrap_lock);
-       unsigned long   flags;
        int             index;
        ktrace_entry_t  *ktep;
 
@@ -161,12 +159,8 @@ ktrace_enter(
        /*
         * Grab an entry by pushing the index up to the next one.
         */
-       spin_lock_irqsave(&wrap_lock, flags);
-       index = ktp->kt_index;
-       if (++ktp->kt_index == ktp->kt_nentries)
-               ktp->kt_index = 0;
-       spin_unlock_irqrestore(&wrap_lock, flags);
-
+       index = atomic_add_return(1, &ktp->kt_index);
+       index = (index - 1) % ktp->kt_nentries;
        if (!ktp->kt_rollover && index == ktp->kt_nentries - 1)
                ktp->kt_rollover = 1;
 
@@ -199,11 +193,12 @@ int
 ktrace_nentries(
        ktrace_t        *ktp)
 {
-       if (ktp == NULL) {
+       int     index;
+       if (ktp == NULL)
                return 0;
-       }
 
-       return (ktp->kt_rollover ? ktp->kt_nentries : ktp->kt_index);
+       index = atomic_read(&ktp->kt_index) % ktp->kt_nentries;
+       return (ktp->kt_rollover ? ktp->kt_nentries : index);
 }
 
 /*
@@ -228,7 +223,7 @@ ktrace_first(ktrace_t   *ktp, ktrace_sna
        int             nentries;
 
        if (ktp->kt_rollover)
-               index = ktp->kt_index;
+               index = atomic_read(&ktp->kt_index) % ktp->kt_nentries;
        else
                index = 0;
 
Index: 2.6.x-xfs-new/fs/xfs/support/ktrace.h
===================================================================
--- 2.6.x-xfs-new.orig/fs/xfs/support/ktrace.h  2007-10-02 16:01:47.000000000 
+1000
+++ 2.6.x-xfs-new/fs/xfs/support/ktrace.h       2008-02-18 15:32:28.911073469 
+1100
@@ -30,7 +30,7 @@ typedef struct ktrace_entry {
  */
 typedef struct ktrace {
        int             kt_nentries;    /* number of entries in trace buf */
-       int             kt_index;       /* current index in entries */
+       atomic_t        kt_index;       /* current index in entries */
        int             kt_rollover;
        ktrace_entry_t  *kt_entries;    /* buffer of entries */
 } ktrace_t;