linux-kernel - Re: [PATCH] ring-buffer: Fix polling on trace

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Message-ID: <20140627065243.GA2826@devbig242.prn2.facebook.com>
Date:	Thu, 26 Jun 2014 23:52:45 -0700
From:	Martin Lau <kafai@...com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	<linux-kernel@...r.kernel.org>, <clm@...com>
Subject: Re: [PATCH] ring-buffer: Fix polling on trace_pipe

Hi Steve,

I retried the test program (with the kernel patch).  It does block from
time to time.  I spotted the ee.events is not set to EPOLLIN before
calling epll_ctl(eft, EPOLL_CTL_ADD,...).  I fixed it and ran
the test in a bash-loop.

I have the kafai-2 version attached (with some more logging in case
if it still blocks).

Can you retry?

Thanks,
Martin

On Thu, Jun 26, 2014 at 08:53:27PM -0400, Steven Rostedt wrote:
> On Thu, 26 Jun 2014 11:34:46 -0700
> Martin Lau <kafai@...com> wrote:
> 
> > Hi Steve,
> > 
> > Can the modified test program reproduce the problem in your test setup?
> 
> Ah sorry, got distracted by other work.
> 
> OK, I just tried it out, and here's my results:
> 
> I ran you code with my current kernel and this is what I got:
> 
> # ./ftrace-test-epoll-kafai 
>            <...>-3183  [002] ...1    81.777891: tracing_mark_write:
> some data 3183: waitting for more data......
> 3184: written more data
> 
> And it just hung there.
> 
> 
> Then I applied your patch, compiled and booted it, and ran the test
> again, and I got this:
> 
> # ./ftrace-test-epoll-kafai
> 
> It just hung there. No forward progress.
> 
> I don't think that was the result you intended.
> 
> -- Steve
> 
> 

View attachment "ftrace-test-epoll-kafai-2.c" of type "text/plain" (2393 bytes)