linux-kernel - Re: perf script: rwtop: SIGALRM and pipe read race

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Date:	Fri, 14 Sep 2012 10:05:03 -0600
From:	David Ahern <dsahern@...il.com>
To:	Andrew Jones <drjones@...hat.com>
CC:	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	paulus@...ba.org, mingo@...hat.com, acme@...stprotocols.net,
	tzanussi@...il.com
Subject: Re: perf script: rwtop: SIGALRM and pipe read race

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 rwtop.pl 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.

David
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