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

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Date:	Mon, 17 Sep 2012 10:32:36 -0600
From:	David Ahern <dsahern@...il.com>
To:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
CC:	Andrew Jones <drjones@...hat.com>, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, paulus@...ba.org, mingo@...hat.com,
	tzanussi@...il.com
Subject: Re: perf script: rwtop: SIGALRM and pipe read race

On 9/17/12 10:02 AM, Arnaldo Carvalho de Melo wrote:
> Em Mon, Sep 17, 2012 at 09:16:19AM -0600, David Ahern escreveu:
>> On 9/17/12 8:55 AM, David Ahern wrote:
>>> 2. the rwtop.pl script is not handling negative return values ($ret < 0)
>>> properly -- the '$ret > 0' check is succeeding even though $ret is
>>> negative (e.g., -EAGAIN) leading to astronomical read values
>>
>> I think perl is treating $ret as an unsigned integer.
>>
>> Again, I know little about perl, but this change to
>> ./scripts/perl/rwtop.pl makes it behave properly:
>>
>>      my $n = sprintf("%d", $ret);
>>
>>      if ($n > 0) {
>>      ...
>
> I think what you figured out makes sense, its the best we have and you
> found it to get it back working, could you please send the two patches
> properly signed-off, etc?

I'll do that for the readn.

The above change seems like a workaround - not a proper fix. Why is perl 
treating it like an unsigned int? The trace format for the read syscall 
shows long. How is the binding done for perl? I can't make heads or 
tails of it scanning the files under perl. If we are forced to do a hack 
like the above is there a better way? I tried (int) $ret and perl did 
not like that. Any other (better) options?

David

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