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

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Message-ID: <20160419172837.GA37697@kafai-mba.local>
Date:	Tue, 19 Apr 2016 10:28:37 -0700
From:	Martin KaFai Lau <kafai@...com>
To:	Soheil Hassas Yeganeh <soheil@...gle.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 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).
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.