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Date: Sat, 15 Apr 2023 16:19:39 +0200
From: Gerhard Engleder <gerhard@...leder-embedded.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc: netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net,
kuba@...nel.org, edumazet@...gle.com, pabeni@...hat.com,
bjorn@...nel.org, magnus.karlsson@...el.com,
jonathan.lemon@...il.com
Subject: Re: [PATCH net-next 4/5] tsnep: Add XDP socket zero-copy RX support
On 07.04.23 15:41, Maciej Fijalkowski wrote:
> On Wed, Apr 05, 2023 at 09:13:58PM +0200, Gerhard Engleder wrote:
>> On 03.04.23 19:26, Maciej Fijalkowski wrote:
>>> On Mon, Apr 03, 2023 at 07:19:15PM +0200, Maciej Fijalkowski wrote:
>>>> On Sun, Apr 02, 2023 at 09:38:37PM +0200, Gerhard Engleder wrote:
>>>>
>>>> Hey Gerhard,
>>>>
>>>>> Add support for XSK zero-copy to RX path. The setup of the XSK pool can
>>>>> be done at runtime. If the netdev is running, then the queue must be
>>>>> disabled and enabled during reconfiguration. This can be done easily
>>>>> with functions introduced in previous commits.
>>>>>
>>>>> A more important property is that, if the netdev is running, then the
>>>>> setup of the XSK pool shall not stop the netdev in case of errors. A
>>>>> broken netdev after a failed XSK pool setup is bad behavior. Therefore,
>>>>> the allocation and setup of resources during XSK pool setup is done only
>>>>> before any queue is disabled. Additionally, freeing and later allocation
>>>>> of resources is eliminated in some cases. Page pool entries are kept for
>>>>> later use. Two memory models are registered in parallel. As a result,
>>>>> the XSK pool setup cannot fail during queue reconfiguration.
>>>>>
>>>>> In contrast to other drivers, XSK pool setup and XDP BPF program setup
>>>>> are separate actions. XSK pool setup can be done without any XDP BPF
>>>>> program. The XDP BPF program can be added, removed or changed without
>>>>> any reconfiguration of the XSK pool.
>>>>
>>>> I won't argue about your design, but I'd be glad if you would present any
>>>> perf numbers (ZC vs copy mode) just to give us some overview how your
>>>> implementation works out. Also, please consider using batching APIs and
>>>> see if this gives you any boost (my assumption is that it would).
>>>>
>>>>>
>>>>> Signed-off-by: Gerhard Engleder <gerhard@...leder-embedded.com>
>>>>> ---
>>>>> drivers/net/ethernet/engleder/tsnep.h | 7 +
>>>>> drivers/net/ethernet/engleder/tsnep_main.c | 432 ++++++++++++++++++++-
>>>>> drivers/net/ethernet/engleder/tsnep_xdp.c | 67 ++++
>>>>> 3 files changed, 488 insertions(+), 18 deletions(-)
>>>
>>> (...)
>>>
>>>>> +
>>>>> static bool tsnep_xdp_run_prog(struct tsnep_rx *rx, struct bpf_prog *prog,
>>>>> struct xdp_buff *xdp, int *status,
>>>>> - struct netdev_queue *tx_nq, struct tsnep_tx *tx)
>>>>> + struct netdev_queue *tx_nq, struct tsnep_tx *tx,
>>>>> + bool zc)
>>>>> {
>>>>> unsigned int length;
>>>>> - unsigned int sync;
>>>>> u32 act;
>>>>> length = xdp->data_end - xdp->data_hard_start - XDP_PACKET_HEADROOM;
>>>>> act = bpf_prog_run_xdp(prog, xdp);
>>>>> -
>>>>> - /* Due xdp_adjust_tail: DMA sync for_device cover max len CPU touch */
>>>>> - sync = xdp->data_end - xdp->data_hard_start - XDP_PACKET_HEADROOM;
>>>>> - sync = max(sync, length);
>>>>> -
>>>>> switch (act) {
>>>>> case XDP_PASS:
>>>>> return false;
>>>>> @@ -1027,8 +1149,21 @@ static bool tsnep_xdp_run_prog(struct tsnep_rx *rx, struct bpf_prog *prog,
>>>>> trace_xdp_exception(rx->adapter->netdev, prog, act);
>>>>> fallthrough;
>>>>> case XDP_DROP:
>>>>> - page_pool_put_page(rx->page_pool, virt_to_head_page(xdp->data),
>>>>> - sync, true);
>>>>> + if (zc) {
>>>>> + xsk_buff_free(xdp);
>>>>> + } else {
>>>>> + unsigned int sync;
>>>>> +
>>>>> + /* Due xdp_adjust_tail: DMA sync for_device cover max
>>>>> + * len CPU touch
>>>>> + */
>>>>> + sync = xdp->data_end - xdp->data_hard_start -
>>>>> + XDP_PACKET_HEADROOM;
>>>>> + sync = max(sync, length);
>>>>> + page_pool_put_page(rx->page_pool,
>>>>> + virt_to_head_page(xdp->data), sync,
>>>>> + true);
>>>>> + }
>>>>> return true;
>>>>> }
>>>>> }
>>>>> @@ -1181,7 +1316,8 @@ static int tsnep_rx_poll(struct tsnep_rx *rx, struct napi_struct *napi,
>>>>> length, false);
>>>>> consume = tsnep_xdp_run_prog(rx, prog, &xdp,
>>>>> - &xdp_status, tx_nq, tx);
>>>>> + &xdp_status, tx_nq, tx,
>>>>> + false);
>>>>> if (consume) {
>>>>> rx->packets++;
>>>>> rx->bytes += length;
>>>>> @@ -1205,6 +1341,125 @@ static int tsnep_rx_poll(struct tsnep_rx *rx, struct napi_struct *napi,
>>>>> return done;
>>>>> }
>>>>> +static int tsnep_rx_poll_zc(struct tsnep_rx *rx, struct napi_struct *napi,
>>>>> + int budget)
>>>>> +{
>>>>> + struct tsnep_rx_entry *entry;
>>>>> + struct netdev_queue *tx_nq;
>>>>> + struct bpf_prog *prog;
>>>>> + struct tsnep_tx *tx;
>>>>> + int desc_available;
>>>>> + int xdp_status = 0;
>>>>> + struct page *page;
>>>>> + int done = 0;
>>>>> + int length;
>>>>> +
>>>>> + desc_available = tsnep_rx_desc_available(rx);
>>>>> + prog = READ_ONCE(rx->adapter->xdp_prog);
>>>>> + if (prog) {
>>>>> + tx_nq = netdev_get_tx_queue(rx->adapter->netdev,
>>>>> + rx->tx_queue_index);
>>>>> + tx = &rx->adapter->tx[rx->tx_queue_index];
>>>>> + }
>>>>> +
>>>>> + while (likely(done < budget) && (rx->read != rx->write)) {
>>>>> + entry = &rx->entry[rx->read];
>>>>> + if ((__le32_to_cpu(entry->desc_wb->properties) &
>>>>> + TSNEP_DESC_OWNER_COUNTER_MASK) !=
>>>>> + (entry->properties & TSNEP_DESC_OWNER_COUNTER_MASK))
>>>>> + break;
>>>>> + done++;
>>>>> +
>>>>> + if (desc_available >= TSNEP_RING_RX_REFILL) {
>>>>> + bool reuse = desc_available >= TSNEP_RING_RX_REUSE;
>>>>> +
>>>>> + desc_available -= tsnep_rx_refill_zc(rx, desc_available,
>>>>> + reuse);
>>>>> + if (!entry->xdp) {
>>>>> + /* buffer has been reused for refill to prevent
>>>>> + * empty RX ring, thus buffer cannot be used for
>>>>> + * RX processing
>>>>> + */
>>>>> + rx->read = (rx->read + 1) % TSNEP_RING_SIZE;
>>>>> + desc_available++;
>>>>> +
>>>>> + rx->dropped++;
>>>>> +
>>>>> + continue;
>>>>> + }
>>>>> + }
>>>>> +
>>>>> + /* descriptor properties shall be read first, because valid data
>>>>> + * is signaled there
>>>>> + */
>>>>> + dma_rmb();
>>>>> +
>>>>> + prefetch(entry->xdp->data);
>>>>> + length = __le32_to_cpu(entry->desc_wb->properties) &
>>>>> + TSNEP_DESC_LENGTH_MASK;
>>>>> + entry->xdp->data_end = entry->xdp->data + length;
>>>>> + xsk_buff_dma_sync_for_cpu(entry->xdp, rx->xsk_pool);
>>>>> +
>>>>> + /* RX metadata with timestamps is in front of actual data,
>>>>> + * subtract metadata size to get length of actual data and
>>>>> + * consider metadata size as offset of actual data during RX
>>>>> + * processing
>>>>> + */
>>>>> + length -= TSNEP_RX_INLINE_METADATA_SIZE;
>>>>> +
>>>>> + rx->read = (rx->read + 1) % TSNEP_RING_SIZE;
>>>>> + desc_available++;
>>>>> +
>>>>> + if (prog) {
>>>>> + bool consume;
>>>>> +
>>>>> + entry->xdp->data += TSNEP_RX_INLINE_METADATA_SIZE;
>>>>> + entry->xdp->data_meta += TSNEP_RX_INLINE_METADATA_SIZE;
>>>>> +
>>>>> + consume = tsnep_xdp_run_prog(rx, prog, entry->xdp,
>>>>> + &xdp_status, tx_nq, tx,
>>>>> + true);
>>>>
>>>> reason for separate xdp run prog routine for ZC was usually "likely-fying"
>>>> XDP_REDIRECT action as this is the main action for AF_XDP which was giving
>>>> us perf improvement. Please try this out on your side to see if this
>>>> yields any positive value.
>>>
>>> One more thing - you have to handle XDP_TX action in a ZC specific way.
>>> Your current code will break if you enable xsk_pool and return XDP_TX from
>>> XDP prog.
>>
>> I took again a look to igc, but I didn't found any specifics for XDP_TX
>> ZC. Only some buffer flipping, which I assume is needed for shared
>> pages.
>> For ice I see a call to xdp_convert_buff_to_frame() in ZC path, which
>> has some XSK logic within. Is this the ZC specific way? igc calls
>> xdp_convert_buff_to_frame() in both cases, so I'm not sure. But I will
>> try the XDP_TX action. I did test only with xdpsock.
>
> I think I will back off a bit with a statement that your XDP_TX is clearly
> broken, here's why.
>
> igc when converting xdp_buff to xdp_frame and xdp_buff's memory being
> backed by xsk_buff_pool will grab the new page from kernel, copy the
> contents of xdp_buff to it, recycle xdp_buff back to xsk_buff_pool and
> return new page back to driver (i have just described what
> xdp_convert_zc_to_xdp_frame() is doing). Thing is that it is expensive and
> hurts perf and we stepped away from this on ice, this is a matter of
> storing the xdp_buff onto adequate Tx buffer struct that you can access
> while cleaning Tx descriptors so that you'll be able to xsk_buff_free()
> it.
>
> So saying 'your current code will break' might have been too much from my
> side. Just make sure that your XDP_TX action on ZC works. In order to do
> that, i was loading xdpsock with XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD flag
> on a queue receiving frames and then running xdp_rxq_info with XDP_TX
> action.
I took a look to the new ice code. It makes sense to prevent that
copying. I took a note for future work. For now XDP_REDIRECT is
the optimisation path.
Tested as suggested. XDP_TX works.
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