netdev - Re: [RFC PATCH v2 net-next 2/7] tcp: Merge tx_flags/tskey/txstamp_ack in tcp_collapse

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Date:	Tue, 19 Apr 2016 13:42:51 -0400
From:	Soheil Hassas Yeganeh <soheil@...gle.com>
To:	Martin KaFai Lau <kafai@...com>
Cc:	netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Soheil Hassas Yeganeh <soheil.kdev@...il.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Kernel Team <kernel-team@...com>
Subject: Re: [RFC PATCH v2 net-next 2/7] tcp: Merge tx_flags/tskey/txstamp_ack
 in tcp_collapse_retrans

On Tue, Apr 19, 2016 at 1:28 PM, Martin KaFai Lau <kafai@...com> wrote:
> On Tue, Apr 19, 2016 at 01:32:14AM -0400, Soheil Hassas Yeganeh wrote:
>> > +               TCP_SKB_CB(skb)->txstamp_ack =
>> > +                       !!(shinfo->tx_flags & SKBTX_ACK_TSTAMP);
>>
>> Maybe we can skip a conditional jump here (because of !!), by simply
>> using the cached bit in next_skb:
>> TCP_SKB_CB(skb)->txstamp_ack = TCP_SKB_CB(next_skb)->txstamp_ack;
> Recall the tx_flags are merged/combined (and so should be the txstamp_ack).

Oh sure, sorry, I missed an "or":

TCP_SKB_CB(skb)->txstamp_ack |= TCP_SKB_CB(next_skb)->txstamp_ack;

> Would there be a case that TCP_SKB_CB(skb)->txstamp_ack is 1 and
> TCP_SKB_CB(next_skb)->txstamp_ack is 0?
>
> I can change it like the following which may help in showing the intention:
> if (TCP_SKB_CB(next_skb)->txstamp_ack)
>         TCP_SKB_CB(skb)->txstamp_ack = 1;
>
> A bit off topic, I feel like the SKBTX_ACK_TSTAMP and txstamp_ack are sort
> of redundant but I have not look into the details yet, so not completely
> sure.  It wwould be a separate cleanup patch if it is the case.

As Eric mentioned, this is needed to avoid a cache-line miss in
accessing the shared info.