netdev - RE: [PATCHv2 net-next 04/12] sctp: implement make_datafrag for sctp_stream

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Date:   Fri, 8 Dec 2017 17:29:00 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Xin Long' <lucien.xin@...il.com>
CC:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        network dev <netdev@...r.kernel.org>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: RE: [PATCHv2 net-next 04/12] sctp: implement make_datafrag for
 sctp_stream_interleave

From: Xin Long ...
> Hi, David, Sorry,  I'm not sure we're worrying about the cpu cost or
> codes style now ?
> 
> For cpu cost,  I think 0x848(%r13) operation must be better than the
> generated code of if-else.

Nope - the call xxx(%ryyy) is likely to be a data cache miss and a complete
cpu pipeline stall.

The conditional will be a data cache hit and the code (for one branch)
will be prefetched and speculatively executed.

Some very modern cpu might manage to predict indirect jumps, but for
most it is a full pipeline stall.

	David