netdev - Re: [PATCH v4 1/5] net-timestamp: COMPLETION timestamp on packet tx completion

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Message-ID: <3059054fea81ac877d36becf46f248a0aae6f4a1.camel@iki.fi>
Date: Sun, 23 Feb 2025 22:52:48 +0000
From: Pauli Virtanen <pav@....fi>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, Jason Xing
	 <kerneljasonxing@...il.com>
Cc: linux-bluetooth@...r.kernel.org, Luiz Augusto von Dentz
	 <luiz.dentz@...il.com>, netdev@...r.kernel.org, davem@...emloft.net, 
	kuba@...nel.org, gerhard@...leder-embedded.com
Subject: Re: [PATCH v4 1/5] net-timestamp: COMPLETION timestamp on packet tx
 completion

Hi,

to, 2025-02-20 kello 18:19 -0500, Willem de Bruijn kirjoitti:
> Willem de Bruijn wrote:
[clip]
> > 
> > Reclaiming space is really up to whoever needs it.
> > 
> > I'll take a quick look, just to see if there is an obvious path and
> > we can postpone this whole conversation to next time we need a bit.
> 
> SKBTX_HW_TSTAMP_USE_CYCLES is only true if SOF_TIMESTAMPING_BIND_PHC.
> It cannot be set per cmsg (is not in SOF_TIMESTAMPING_TX_RECORD_MASK),
> so no need to record it per skb.
> 
> It only has two drivers using it, which can easily be updated:
> 
> 	-                if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP_USE_CYCLES)
> 	+                if (skb->sk &&
> 	+                    READ_ONCE(sk->sk_tsflags) & SOF_TIMESTAMPING_BIND_PHC)
> 					tx_flags |= IGC_TX_FLAGS_TSTAMP_TIMER_1;
> 
> They later call skb_tstamp_tx, which does nothing if !skb->sk.
> Only cost is a higher cost of accessing the sk cacheline.
> 
> SKBTX_WIFI_STATUS essentially follows the same argument. It can only
> be set in the sockopt. It has a handful more callsites that would need
> to be updated. sock_flag(sk, SOCK_WIFI_STATUS) will be tested without
> the socket lock held. But this is already the case in the UDP lockless
> fast path through ip_make_skb.
> 
> SKBTX_HW_TSTAMP_NETDEV is only used on Rx. Could shadow another bit
> that is used only on Tx.
> 
> SKBTX_IN_PROGRESS is only used by the driver to suppress the software
> tx timestamp from skb_tx_timestamp if a later hardware timestamp will
> be generated. Predates SOF_TIMESTAMPING_OPT_TX_SWHW.
> 
> In short plenty of bits we can reclaim if we try.
> 
> SKBTX_BPF was just merged, so we will have to reclaim one. The first
> one seems most straightforward.

Ok, I'll go back to tx_flags bit for v5 (= v3 + the minor cleanups),
going on top of "net: skb: free up one bit in tx_flags" then.

-- 
Pauli Virtanen