netdev - Re: [PATCH] tcp: splice as many packets as possible at once

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Message-ID: <20090111160548.GA362@ioremap.net>
Date:	Sun, 11 Jan 2009 19:05:48 +0300
From:	Evgeniy Polyakov <zbr@...emap.net>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Willy Tarreau <w@....eu>, David Miller <davem@...emloft.net>,
	ben@...s.com, jarkao2@...il.com, mingo@...e.hu,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	jens.axboe@...cle.com
Subject: Re: [PATCH] tcp: splice as many packets as possible at once

On Sun, Jan 11, 2009 at 05:00:37PM +0100, Eric Dumazet (dada1@...mosbay.com) wrote:
> >>>> 1) the release_sock/lock_sock done in tcp_splice_read() is not necessary
> >>>> to process backlog. Its already done in skb_splice_bits()
> >>> Yes, in the tcp_splice_read() they are added to remove a deadlock.
> >> Could you elaborate ? A deadlock only if !SPLICE_F_NONBLOCK ?
> > 
> > Sorry, I meant that we drop lock in skb_splice_bits() to prevent the deadlock,
> > and tcp_splice_read() needs it to process the backlog.
> 
> While we drop lock in skb_splice_bits() to prevent the deadlock, we
> also process backlog at this stage. No need to process backlog
> again in the higher level function.

Yes, but having it earlier allows to receive new skb while processing
already received.

> > I think that even with non-blocking splice that release_sock/lock_sock
> > is needed, since we are able to do a parallel job: to receive new data
> > (scheduled by early release_sock backlog processing) in bh and to
> > process already received data via splice codepath.
> > Maybe in non-blocking splice mode this is not an issue though, but for
> > the blocking mode this allows to grab more skbs at once in skb_splice_bits.
> 
> skb_splice_bits() operates on one skb, you lost me :)

Exactly, and to have it we earlier release a socket so that it could be
acked and while we copy it or doing anything else, the next one would
received.

-- 
	Evgeniy Polyakov
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