Re: [PATCH net-next v3 00/21] rxrpc: Miscellaneous changes and make use of MSG_SPLICE_PAGES

Simon Horman
2024-03-07T09:39:08Z

On Wed, Mar 06, 2024 at 12:06:30AM +0000, David Howells wrote:
> Here are some changes to AF_RXRPC:
>
> (1) Cache the transmission serial number of ACK and DATA packets in the
> rxrpc_txbuf struct and log this in the retransmit tracepoint.
>
> (2) Don't use atomics on rxrpc_txbuf::flags[*] and cache the intended wire
> header flags there too to avoid duplication.
>
> (3) Cache the wire checksum in rxrpc_txbuf to make it easier to create
> jumbo packets in future (which will require altering the wire header
> to a jumbo header and restoring it back again for retransmission).
>
> (4) Fix the protocol names in the wire ACK trailer struct.
>
> (5) Strip all the barriers and atomics out of the call timer tracking[*].
>
> (6) Remove atomic handling from call->tx_transmitted and
> call->acks_prev_seq[*].
>
> (7) Don't bother resetting the DF flag after UDP packet transmission. To
> change it, we now call directly into UDP code, so it's quick just to
> set it every time.
>
> (8) Merge together the DF/non-DF branches of the DATA transmission to
> reduce duplication in the code.
>
> (9) Add a kvec array into rxrpc_txbuf and start moving things over to it.
> This paves the way for using page frags.
>
> (10) Split (sub)packet preparation and timestamping out of the DATA
> transmission function. This helps pave the way for future jumbo
> packet generation.
>
> (11) In rxkad, don't pick values out of the wire header stored in
> rxrpc_txbuf, buf rather find them elsewhere so we can remove the wire
> header from there.
>
> (12) Move rxrpc_send_ACK() to output.c so that it can be merged with
> rxrpc_send_ack_packet().
>
> (13) Use rxrpc_txbuf::kvec[0] to access the wire header for the packet
> rather than directly accessing the copy in rxrpc_txbuf. This will
> allow that to be removed to a page frag.
>
> (14) Switch from keeping the transmission buffers in rxrpc_txbuf allocated
> in the slab to allocating them using page fragment allocators. There
> are separate allocators for DATA packets (which persist for a while)
> and control packets (which are discarded immediately).
>
> We can then turn on MSG_SPLICE_PAGES when transmitting DATA and ACK
> packets.
>
> We can also get rid of the RCU cleanup on rxrpc_txbufs, preferring
> instead to release the page frags as soon as possible.
>
> (15) Parse received packets before handling timeouts as the former may
> reset the latter.
>
> (16) Make sure we don't retransmit DATA packets after all the packets have
> been ACK'd.
>
> (17) Differentiate traces for PING ACK transmission.
>
> (18) Switch to keeping timeouts as ktime_t rather than a number of jiffies
> as the latter is too coarse a granularity. Only set the call timer at
> the end of the call event function from the aggregate of all the
> timeouts, thereby reducing the number of timer calls made. In future,
> it might be possible to reduce the number of timers from one per call
> to one per I/O thread and to use a high-precision timer.
>
> (19) Record RTT probes after successful transmission rather than recording
> it before and then cancelling it after if unsuccessful[*]. This
> allows a number of calls to get the current time to be removed.
>
> (20) Clean up the resend algorithm as there's now no need to walk the
> transmission buffer under lock[*]. DATA packets can be retransmitted
> as soon as they're found rather than being queued up and transmitted
> when the locked is dropped.
>
> (21) When initially parsing a received ACK packet, extract some of the
> fields from the ack info to the skbuff private data. This makes it
> easier to do path MTU discovery in the future when the call to which a
> PING RESPONSE ACK refers has been deallocated.
>
>
> [*] Possible with the move of almost all code from softirq context to the
> I/O thread.
>
> The patches are tagged here:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git tags/rxrpc-iothread-20240305
>
> And can be found on this branch:
>
> http://git.kernel.org/cgit/linux/kernel/git/dhowells/linux-fs.git/log/?h=rxrpc-iothread
>
> David
>
> Link: https://lore.kernel.org/r/20240301163807.385573-1-dhowells@redhat.com/ # v1
> Link: https://lore.kernel.org/r/20240304084322.705539-1-dhowells@redhat.com/ # v2
>
> Changes
> =======
> ver #3)
> - Use passed-in gfp in rxkad_alloc_txbuf() rather than GFP_KRNEL.

Hi David,

Thanks for the update above.
For the record, I don't have anything to flag in this revision of the patchset.

> - Adjust rxkad_alloc_txbuf()'s txb check to put return in if-statement.
>
> ver #2)
> - Removed an unused variable.
> - Use ktime_to_us() rather than dividing a ktime by 1000 in tracepoints.

...