perf diff: Percent calcs should use double values

Otherwise we do integer math and the delta values round up to
multiples of 1.0%.

Also, calculate absolute values. Things look precise now:

$ perf report -i perf.data.old --sort dso,symbol | head -13
     9.02%  libc-2.10.1.so               [.] _IO_vfprintf_internal
     4.88%  find                         [.] 0x00000000014af0
     2.91%  [kernel]                     [k] __kmalloc
     2.85%  [kernel]                     [k] ext4_htree_store_dirent
     2.50%  libc-2.10.1.so               [.] __GI_memmove
     2.44%  [kernel]                     [k] half_md4_transform
     2.43%  [kernel]                     [k] _spin_lock
     2.33%  [kernel]                     [k] system_call
$ perf report -i perf.data --sort dso,symbol | head -13
     8.55%  libc-2.10.1.so               [.] _IO_vfprintf_internal
     3.11%  [kernel]                     [k] __kmalloc
     3.07%  [kernel]                     [k] ext4_htree_store_dirent
     2.66%  find                         [.] 0x00000000016bcf
     2.61%  [kernel]                     [k] _atomic_dec_and_lock
     2.46%  [kernel]                     [k] half_md4_transform
     2.41%  libc-2.10.1.so               [.] __GI_memmove
     2.30%  find                         [.] 0x00000000009219
$ perf diff | head -13
     9.02%     -0.47%  libc-2.10.1.so               [.] _IO_vfprintf_internal
     2.91%     +0.20%  [kernel]                     [k] __kmalloc
     2.85%     +0.23%  [kernel]                     [k] ext4_htree_store_dirent
     1.99%     +0.62%  [kernel]                     [k] _atomic_dec_and_lock
     2.44%     +0.02%  [kernel]                     [k] half_md4_transform
     2.50%     -0.09%  libc-2.10.1.so               [.] __GI_memmove
     1.88%     +0.01%  [kernel]                     [k] __d_lookup
     2.43%     -0.75%  [kernel]                     [k] _spin_lock
     0.97%     +0.62%  [kernel]                     [k] path_get
     1.99%     -0.42%  libc-2.10.1.so               [.] _int_malloc
$
Signed-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1260981109-2621-1-git-send-email-acme@infradead.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

tools/perf/util/hist.c

 #include "hist.h"
 #include "session.h"
 #include "sort.h"
+#include <math.h>
 
 struct callchain_param	callchain_param = {
 	.mode	= CHAIN_GRAPH_REL,
@@ -494,13 +495,13 @@ static size_t hist_entry__fprintf(struct hist_entry *self,
 		double old_percent = 0, new_percent = 0, diff;
 
 		if (total > 0)
-			old_percent = (count * 100) / total;
+			old_percent = (count * 100.0) / total;
 		if (session->events_stats.total > 0)
-			new_percent = (self->count * 100) / session->events_stats.total;
+			new_percent = (self->count * 100.0) / session->events_stats.total;
 
-		diff = old_percent - new_percent;
+		diff = new_percent - old_percent;
 
-		if ((u64)diff != 0)
+		if (fabs(diff) >= 0.01)
 			snprintf(bf, sizeof(bf), "%+4.2F%%", diff);
 		else
 			snprintf(bf, sizeof(bf), " ");