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Date:	Wed, 15 Aug 2012 16:45:04 -0400
From:	"Atchley, Scott" <atchleyes@...l.gov>
To:	Sage Weil <sage@...tank.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>
Subject: Re: regression with poll(2)?

On Aug 15, 2012, at 3:46 PM, Sage Weil wrote:

> I'm experiencing a stall with Ceph daemons communicating over TCP that 
> occurs reliably with 3.6-rc1 (and linus/master) but not 3.5.  The basic 
> situation is:
> 
> - the socket is two processes communicating over TCP on the same host, e.g. 
> 
> tcp        0 2164849 10.214.132.38:6801      10.214.132.38:51729     ESTABLISHED
> 
> - one end writes a bunch of data in
> - the other end consumes data, but at some point stalls.
> - reads are nonblocking, e.g.
> 
>  int got = ::recv( sd, buf, len, MSG_DONTWAIT );
> 
> and between those calls we wait with
> 
>  struct pollfd pfd;
>  short evmask;
>  pfd.fd = sd;
>  pfd.events = POLLIN;
> #if defined(__linux__)
>  pfd.events |= POLLRDHUP;
> #endif
> 
>  if (poll(&pfd, 1, msgr->timeout) <= 0)
>    return -1;
> 
> - in my case the timeout is ~15 minutes.  at that point it errors out, 
> and the daemons reconnect and continue for a while until hitting this 
> again.
> 
> - at the time of the stall, the reading process is blocked on that 
> poll(2) call.  There are a bunch of threads stuck on poll(2), some of them 
> stuck and some not, but they all have stacks like
> 
> [<ffffffff8118f6f9>] poll_schedule_timeout+0x49/0x70
> [<ffffffff81190baf>] do_sys_poll+0x35f/0x4c0
> [<ffffffff81190deb>] sys_poll+0x6b/0x100
> [<ffffffff8163d369>] system_call_fastpath+0x16/0x1b
> 
> - you'll note that the netstat output shows data queued:
> 
> tcp        0 1163264 10.214.132.36:6807      10.214.132.36:41738     ESTABLISHED
> tcp        0 1622016 10.214.132.36:41738     10.214.132.36:6807      ESTABLISHED
> 
> etc.
> 
> Is this a known regression?  Or might I be misusing the API?  What 
> information would help track it down?
> 
> Thanks!
> sage


Sage,

Do you see the same behavior when using two hosts (i.e. not loopback)? If different, how much data is in the pipe in the localhost case?

Scott


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