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Date:	Tue, 14 Oct 2014 15:30:59 -0400 (EDT)
From:	David Miller <>
Subject: Re: [PATCH net] tcp: fix tcp_ack() performance problem

From: Eric Dumazet <>
Date: Sat, 11 Oct 2014 15:17:29 -0700

> From: Eric Dumazet <>
> We worked hard to improve tcp_ack() performance, by not accessing
> skb_shinfo() in fast path (cd7d8498c9a5 tcp: change tcp_skb_pcount()
> location)
> We still have one spurious access because of ACK timestamping,
> added in commit e1c8a607b281 ("net-timestamp: ACK timestamp for
> bytestreams")
> By checking if sk_tsflags has SOF_TIMESTAMPING_TX_ACK set,
> we can avoid two cache line misses for the common case.
> While we are at it, add two prefetchw() :
> One in tcp_ack() to bring skb at the head of write queue.
> One in tcp_clean_rtx_queue() loop to bring following skb,
> as we will delete skb from the write queue and dirty skb->next->prev.
> Add a couple of [un]likely() clauses.
> After this patch, tcp_ack() is no longer the most consuming
> function in tcp stack.
> Signed-off-by: Eric Dumazet <>

Applied, thanks.
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