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: <CAF=yD-K-7h36Kw1dirPVhsbsv63dJSbLoefUiNzh_UvaK7_UXg@mail.gmail.com>
Date:	Thu, 21 Apr 2016 16:25:55 -0400
From:	Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:	Soheil Hassas Yeganeh <soheil@...gle.com>
Cc:	Martin KaFai Lau <kafai@...com>,
	Eric Dumazet <edumazet@...gle.com>,
	netdev <netdev@...r.kernel.org>,
	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 2:24 PM, Soheil Hassas Yeganeh
<soheil@...gle.com> wrote:
> On Tue, Apr 19, 2016 at 2:18 PM, Martin KaFai Lau <kafai@...com> wrote:
>> On Tue, Apr 19, 2016 at 10:35:52AM -0700, Eric Dumazet wrote:
>>> On Tue, Apr 19, 2016 at 10:28 AM, Martin KaFai Lau <kafai@...com> wrote:
>>>
>>> > 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.

Yes, with the introduction of txstamp_ack, SKBTX_ACK_TSTAMP is completely
redundant.

>>>
>>> Please read 6b084928baac562ed61866f540a96120e9c9ddb7 changelog ;)
>>>
>>> A cache line miss avoidance is critical
>> I looked at the patch but I probably am missing something :(
>> Is checking txstamp_ack alone enough and SKBTX_ACK_TSTAMP is not needed
>> since they are always set together?
>
> That's right, the check on "(shinfo->tx_flags & SKBTX_ACK_TSTAMP)" in
> tcp_ack_tstamp() is redundant and I had a patch prepared to remove it.

You can even remove the flag completely and

-               tcb->txstamp_ack = !!(shinfo->tx_flags & SKBTX_ACK_TSTAMP);
+               if (tsflags & SOF_TIMESTAMPING_TX_ACK)
+                       tcb->txstamp_ack = 1;

> But I thought it's better to wait for
> https://patchwork.ozlabs.org/patch/611938/ to be merged first.
>
> Feel free to remove it in your patches, if you'd prefer that.