tmpfs: add accurate compare function to percpu_counter library

Add percpu_counter_compare that allows for a quick but accurate comparison
of percpu_counter with a given value.

A rough count is provided by the count field in percpu_counter structure,
without accounting for the other values stored in individual cpu counters.

The actual count is a sum of count and the cpu counters.  However, count
field is never different from the actual value by a factor of
batch*num_online_cpu.  We do not need to get actual count for comparison
if count is different from the given value by this factor and allows for
quick comparison without summing up all the per cpu counters.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 27 insertions, 0 deletions

diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index aeaa6d734447..ec9048e74f44 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -137,6 +137,33 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
         return NOTIFY_OK;
 }
 
+/*
+ * Compare counter against given value.
+ * Return 1 if greater, 0 if equal and -1 if less
+ */
+int percpu_counter_compare(struct percpu_counter *fbc, s64 rhs)
+{
+        s64     count;
+
+        count = percpu_counter_read(fbc);
+        /* Check to see if rough count will be sufficient for comparison */
+        if (abs(count - rhs) > (percpu_counter_batch*num_online_cpus())) {
+                if (count > rhs)
+                        return 1;
+                else
+                        return -1;
+        }
+        /* Need to use precise count */
+        count = percpu_counter_sum(fbc);
+        if (count > rhs)
+                return 1;
+        else if (count < rhs)
+                return -1;
+        else
+                return 0;
+}
+EXPORT_SYMBOL(percpu_counter_compare);
+
 static int __init percpu_counter_startup(void)
 {
         compute_batch_value();