netdev - Re: [PATCH 2.6.23-rc6 Resending] NETWORKING : Edge Triggered EPOLLOUT events get missed for TCP sockets

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Date:	Thu, 20 Sep 2007 08:11:48 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Nagendra Tomar <tomer_iisc@...oo.com>
CC:	Davide Libenzi <davidel@...ilserver.org>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2.6.23-rc6 Resending] NETWORKING : Edge Triggered EPOLLOUT
 events get missed for TCP sockets

Nagendra Tomar a écrit :
> --- Davide Libenzi <davidel@...ilserver.org> wrote:
> 
>> On Wed, 19 Sep 2007, David Miller wrote:
>>
>>> From: Nagendra Tomar <tomer_iisc@...oo.com>
>>> Date: Wed, 19 Sep 2007 15:37:09 -0700 (PDT)
>>>
>>>> With the SOCK_NOSPACE check in tcp_check_space(), this epoll_wait call will 
>>>> not return, even when the incoming acks free the buffers.
>>>>         Note that this patch assumes that the SOCK_NOSPACE check in
>>>> tcp_check_space is a trivial optimization which can be safely removed.
>>> I already replied to your patch posting explaining that whatever is
>>> not setting SOCK_NOSPACE should be fixed instead.
>>>
>>> Please address that, thanks.
>> You're not planning of putting the notion of a SOCK_NOSPACE bit inside a 
>> completely device-unaware interface like epoll, I hope?
>>
> 
> Definitely not ! 
> 
> The point is that the "tcp write space available" 
> wakeup does not get called if SOCK_NOSPACE bit is not set. This was
> fine when the wakeup was merely a wakeup (since SOCK_NOSPACE bit 
> indicated that someone really cared abt the wakeup). Now after the
> introduction of callback'ed wakeups, we might have some work to
> do inside the callback even if there is nobody interested in the wakeup
> at that point of time. 
> 
> In this particular case the ep_poll_callback is not getting called and
> hence the socket fd is not getting added to the ready list.
> 

Does it means that with your patch each ACK on a ET managed socket will 
trigger an epoll event   ?

Maybe your very sensitive high throuput appication needs to set a flag or 
something at socket level to ask for such a behavior.

The default should stay as is. That is an event should be sent only if someone 
cared about the wakeup.

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