linux-kernel - Re: [RFC][PATCH][NET] Fix never pruned tcp out-of-order queue

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Message-ID: <20080415115924.GA1550@ms2.inr.ac.ru>
Date:	Tue, 15 Apr 2008 15:59:24 +0400
From:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Vitaliy Gusev <vgusev@...nvz.org>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH][NET] Fix never pruned tcp out-of-order queue

Hello!

> I still think the guards are pretty much the same as before, sorry:)

Guards inside tcp_prune_queue() are the same exactly.

But the patch adds the second point where out-of-order queue is discarded.
It is when the socket is under rcvbuf, but nevertheless skb cannot
be queued due to system-wide limit. In that case out-of-order queue
is dropped and the limits are rechecked.

> But why not repeat the whole prune for all cases in this case then?

Collapsing and tuning rcv_ssthresh was done once, they are not guarded
by rcvbuf check. So, repeating those steps would be useless.

The only thing is:

> e.g. you should probably at least repeat the third step (setting
> pred_flags to 0) too.

Formally, this is correct. But this is not necessary, pred_flags reset
is redundant even in the first place. The fast path is not so fast,
memory limit is checked explicitly there.

The patch is not perfect. F.e. tcp_prune_ofo_queue() could see empty
out-of-order queue, in this case the second sk_stream_rmem_schedule()
is useless and could be skipped. But it is the second order effect.

I think this will work.

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