netdev - Re: [PATCH v2 net-next 3/8] tcp: use one bit in TCP_SKB

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Date:	Fri, 01 Apr 2016 08:44:58 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Soheil Hassas Yeganeh <soheil.kdev@...il.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, willemb@...gle.com,
	edumazet@...gle.com, ycheng@...gle.com, ncardwell@...gle.com,
	kafai@...com, Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [PATCH v2 net-next 3/8] tcp: use one bit in TCP_SKB_CB to mark
 ACK timestamps

On Fri, 2016-04-01 at 11:04 -0400, Soheil Hassas Yeganeh wrote:
> From: Soheil Hassas Yeganeh <soheil@...gle.com>
> 
> Currently, to avoid a cache line miss for accessing skb_shinfo,
> tcp_ack_tstamp skips socket that do not have
> SOF_TIMESTAMPING_TX_ACK bit set in sk_tsflags. This is
> implemented based on an implicit assumption that the
> SOF_TIMESTAMPING_TX_ACK is set via socket options for the
> duration that ACK timestamps are needed.
> 
> To implement per-write timestamps, this check should be
> removed and replaced with a per-packet alternative that
> quickly skips packets missing ACK timestamps marks without
> a cache-line miss.
> 
> To enable per-packet marking without a cache line miss, use
> one bit in TCP_SKB_CB to mark a whether a SKB might need a
> ack tx timestamp or not. Further checks in tcp_ack_tstamp are not
> modified and work as before.

Acked-by: Eric Dumazet <edumazet@...gle.com>