linux-kernel - Re: ppoll() stuck on POLLIN while TCP peer is sending

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Date:	Thu, 3 Jan 2013 18:32:51 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: ppoll() stuck on POLLIN while TCP peer is sending

Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2013-01-02 at 20:47 +0000, Eric Wong wrote:
> > Eric Wong <normalperson@...t.net> wrote:
> > > [1] my full setup is very strange.
> > > 
> > >     Other than the FUSE component I forgot to mention, little depends on
> > >     the kernel.  With all this, the standalone toosleepy can get stuck.
> > >     I'll try to reproduce it with less...
> > 
> > I just confirmed my toosleepy processes will get stuck while just
> > doing "rsync -a" between local disks.  So this does not depend on
> > sendfile or FUSE to reproduce.
> > --
> 
> How do you tell your 'toosleepy' is stuck ?

My original post showed it stuck with strace (in ppoll + send).
I only strace after seeing it's not using any CPU in top.

http://mid.gmane.org/20121228014503.GA5017@dcvr.yhbt.net
(lsof also confirmed the ppoll/send sockets were peers)

> If reading its output, you should change its logic, there is no
> guarantee the recv() will deliver exactly 16384 bytes each round.
> 
> With the following patch, I cant reproduce the 'apparent stuck'

Right, the output is just an approximation and the logic there
was bogus.

Thanks for looking at this.
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