linux-kernel - Re: [-next regression] TCP window full with EPOLLWAKEUP

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Date:	Mon, 21 May 2012 15:55:18 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
CC:	Arve Hjønnevåg <arve@...roid.com>,
	NeilBrown <neilb@...e.de>, LKML <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jirislaby@...il.com>,
	Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [-next regression] TCP window full with EPOLLWAKEUP

On 05/20/2012 08:34 PM, Rafael J. Wysocki wrote:
> On Sunday, May 20, 2012, Rafael J. Wysocki wrote:
>> On Sunday, May 20, 2012, Rafael J. Wysocki wrote:
>>> On Sunday, May 20, 2012, Jiri Slaby wrote:
>>>> Hi,
>>>>
>>>> a bisection shows that with the following commit from -next:
>>>> commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
>>>> Author: Arve Hjønnevåg <arve@...roid.com>
>>>> Date:   Tue May 1 21:33:34 2012 +0200
>>>>
>>>>     epoll: Add a flag, EPOLLWAKEUP, to prevent suspend while epoll
>>>> events are ready
>>>>
>>>> ====
>>>>
>>>> one of mono programs I use stops receiving data from the network.
>>>> Wireshark shows that the TCP window of a connection is filled. This
>>>> means the program does not read the data fast enough after requesting
>>>> the data.
>>>>
>>>> If I revert that commit on the top of -next (20120518), everything works
>>>> as expected.
>>>
>>> Hmm.  I suppose that the failing program doesn't set EPOLLWAKEUP by mistake,
>>> does it?
>>
>> If it doesn't, we can assume that epi-ws is always NULL and all of the added
>> overhead comes from the function calls.  So, I wonder if the appended patch
>> makes any difference?
> 
> Having thought more about this I have to say this doesn't seem to make much
> sense, because in that case you'd see some progress, although probably a bit
> slower than before.
> 
> So, I think what happens is that the application tries to set EPOLLWAKEUP,
> but doesn't have the capability, so the entire operation fails for it, but
> it doesn't check the return value.
> 
> I wonder if the following helps, then.
> 
> Thanks,
> Rafael
> 
> 
> ---
>  fs/eventpoll.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux/fs/eventpoll.c
> ===================================================================
> --- linux.orig/fs/eventpoll.c
> +++ linux/fs/eventpoll.c
> @@ -1711,7 +1711,7 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, in
>  
>  	/* Check if EPOLLWAKEUP is allowed */
>  	if ((epds.events & EPOLLWAKEUP) && !capable(CAP_EPOLLWAKEUP))
> -		goto error_tgt_fput;
> +		epds.events &= ~EPOLLWAKEUP;

Yes, this fixed the issue.

thanks,
-- 
js
suse labs


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