netdev - Re: [patch 1/4] - Potential performance bottleneck for Linxu TCP

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Message-Id: <20061129.171316.70216831.davem@davemloft.net>
Date:	Wed, 29 Nov 2006 17:13:16 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	akpm@...l.org
Cc:	wenji@...l.gov, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/4] - Potential performance bottleneck for Linxu TCP

From: Andrew Morton <akpm@...l.org>
Date: Wed, 29 Nov 2006 17:08:35 -0800

> On Wed, 29 Nov 2006 16:53:11 -0800 (PST)
> David Miller <davem@...emloft.net> wrote:
> 
> > 
> > Please, it is very difficult to review your work the way you have
> > submitted this patch as a set of 4 patches.  These patches have not
> > been split up "logically", but rather they have been split up "per
> > file" with the same exact changelog message in each patch posting.
> > This is very clumsy, and impossible to review, and wastes a lot of
> > mailing list bandwith.
> > 
> > We have an excellent file, called Documentation/SubmittingPatches, in
> > the kernel source tree, which explains exactly how to do this
> > correctly.
> > 
> > By splitting your patch into 4 patches, one for each file touched,
> > it is impossible to review your patch as a logical whole.
> > 
> > Please also provide your patch inline so people can just hit reply
> > in their mail reader client to quote your patch and comment on it.
> > This is impossible with the attachments you've used.
> > 
> 
> Here you go - joined up, cleaned up, ported to mainline and test-compiled.
> 
> That yield() will need to be removed - yield()'s behaviour is truly awful
> if the system is otherwise busy.  What is it there for?

What about simply turning off CONFIG_PREEMPT to fix this "problem"?

We always properly run the backlog (by doing a release_sock()) before
going to sleep otherwise except for the specific case of taking a page
fault during the copy to userspace.  It is only CONFIG_PREEMPT that
can cause this situation to occur in other circumstances as far as I
can see.

We could also pepper tcp_recvmsg() with some very carefully placed
preemption disable/enable calls to deal with this even with
CONFIG_PREEMPT enabled.
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