[PATCH 3.2 071/107] perf header: Fixup reading of HEADER_NRCPUS feature

[Ben Hutchings]

3.2.72-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>

commit caa470475d9b59eeff093ae650800d34612c4379 upstream.

The original patch introducing this header wrote the number of CPUs available
and online in one order and then swapped those values when reading, fix it.

Before:

  # perf record usleep 1
  # perf report --header-only | grep 'nrcpus \(online\|avail\)'
  # nrcpus online : 4
  # nrcpus avail : 4
  # echo 0 > /sys/devices/system/cpu/cpu2/online
  # perf record usleep 1
  # perf report --header-only | grep 'nrcpus \(online\|avail\)'
  # nrcpus online : 4
  # nrcpus avail : 3
  # echo 0 > /sys/devices/system/cpu/cpu1/online
  # perf record usleep 1
  # perf report --header-only | grep 'nrcpus \(online\|avail\)'
  # nrcpus online : 4
  # nrcpus avail : 2

After the fix, bringing back the CPUs online:

  # perf report --header-only | grep 'nrcpus \(online\|avail\)'
  # nrcpus online : 2
  # nrcpus avail : 4
  # echo 1 > /sys/devices/system/cpu/cpu2/online
  # perf record usleep 1
  # perf report --header-only | grep 'nrcpus \(online\|avail\)'
  # nrcpus online : 3
  # nrcpus avail : 4
  # echo 1 > /sys/devices/system/cpu/cpu1/online
  # perf record usleep 1
  # perf report --header-only | grep 'nrcpus \(online\|avail\)'
  # nrcpus online : 4
  # nrcpus avail : 4

Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>
Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: David Ahern <dsahern@xxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Kan Liang <kan.liang@xxxxxxxxx>
Cc: Stephane Eranian <eranian@xxxxxxxxxx>
Cc: Wang Nan <wangnan0@xxxxxxxxxx>
Fixes: fbe96f29ce4b ("perf tools: Make perf.data more self-descriptive (v8)")
Link: http://lkml.kernel.org/r/20150911153323.GP23511@xxxxxxxxxx
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
[bwh: Backported to 3.2: print_nrcpus() reads and prints these fields
 immediately, so read both of them into an array before printing them in
 reverse order.]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 tools/perf/util/header.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -796,25 +796,19 @@ static void print_cpudesc(struct perf_he
 static void print_nrcpus(struct perf_header *ph, int fd, FILE *fp)
 {
 	ssize_t ret;
-	u32 nr;
+	u32 nr[2];
 
 	ret = read(fd, &nr, sizeof(nr));
 	if (ret != (ssize_t)sizeof(nr))
-		nr = -1; /* interpreted as error */
+		nr[0] = nr[1] = -1; /* interpreted as error */
 
-	if (ph->needs_swap)
-		nr = bswap_32(nr);
+	if (ph->needs_swap) {
+		nr[0] = bswap_32(nr[0]);
+		nr[1] = bswap_32(nr[1]);
+	}
 
-	fprintf(fp, "# nrcpus online : %u\n", nr);
-
-	ret = read(fd, &nr, sizeof(nr));
-	if (ret != (ssize_t)sizeof(nr))
-		nr = -1; /* interpreted as error */
-
-	if (ph->needs_swap)
-		nr = bswap_32(nr);
-
-	fprintf(fp, "# nrcpus avail : %u\n", nr);
+	fprintf(fp, "# nrcpus online : %u\n", nr[1]);
+	fprintf(fp, "# nrcpus avail : %u\n", nr[0]);
 }
 
 static void print_version(struct perf_header *ph, int fd, FILE *fp)

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