Re: perf script: rwtop: SIGALRM and pipe read race

From: David Ahern
Date: Fri Sep 14 2012 - 12:05:07 EST

On 9/14/12 9:39 AM, Andrew Jones wrote:

I recently tried 'perf script rwtop', and it immediately failed with
'failed to read event header'. Running it through strace I found that the
when is reading from the pipe, and gets one of it's alarms, that
the ERESTARTSYS seems to confuse it - causing it to fail. It also appears
that the problem only happens early in execution, or not at all. If I get
lucky and don't hit the problem right away, then rwtop will run fine as
long as I want, without any ERESTARTSYS's in its trace. I also found that
I can avoid hitting the problem by throwing a 'pv -q' in front of the perf
command in tools/perf/scripts/perl/bin/rwtop-report. Which I guess slows
things down in the reader enough to always avoid the race.

Sorry I don't have a solution (patch). I'll look at it more as time
permits, but I thought I'd get it reported for starters though.

This fixes the run-time problem:

diff --git a/tools/perf/util/util.c b/tools/perf/util/util.c
index 1b8775c..a4371ae 100644
--- a/tools/perf/util/util.c
+++ b/tools/perf/util/util.c
@@ -142,6 +142,9 @@ int readn(int fd, void *buf, size_t n)
while (n) {
int ret = read(fd, buf, n);

+ if ((ret < 0) && (errno == EINTR))
+ continue;
if (ret <= 0)
return ret;

The only problem you will find with rwtop is that bytes_read will be really whacky. I traced it to:

if ($ret > 0) {
printf("comm %s bytes_read %d\n", $common_comm, $ret);
$reads{$common_pid}{bytes_read} += $ret;

Somehow the $ret > 0 is passing when in fact it is negative. I do not know much about perl to fix it.

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