On 2021/5/12 7:30, Jakub Kicinski wrote:
On Tue, 11 May 2021 20:13:56 +0800 Yunsheng Lin wrote:
On 2021/5/11 17:04, Yunsheng Lin wrote:
On 2021/5/11 12:22, Jakub Kicinski wrote:
The queues are woken asynchronously without holding any locks via netif_tx_wake_queue(). Theoretically we can have a situation where:
CPU 0 CPU 1 . . dequeue_skb() . netif_xmit_frozen..() # true . . [IRQ] . netif_tx_wake_queue() . <end of IRQ> . netif_tx_action() . set MISSED clear MISSED return NULL ret from qdisc_restart() ret from __qdisc_run() qdisc_run_end()
[...]
Yes, the above does seems to have the above data race.
As my understanding, there is two ways to fix the above data race:
- do not clear the STATE_MISSED for netif_xmit_frozen_or_stopped() case, just check the netif_xmit_frozen_or_stopped() before calling __netif_schedule() at the end of qdisc_run_end(). This seems to only work with qdisc with TCQ_F_ONETXQUEUE flag because it seems we can only check the netif_xmit_frozen_or_stopped() with q->dev_queue, I am not sure q->dev_queue is pointint to which netdev queue when qdisc is not set with TCQ_F_ONETXQUEUE flag.
Isn't the case where we have a NOLOCK qdisc without TCQ_F_ONETXQUEUE rather unexpected? It'd have to be a single pfifo on multi-queue netdev, right? Sounds not worth optimizing for. How about:
static inline void qdisc_run_end(struct Qdisc *qdisc) { write_seqcount_end(&qdisc->running); if (qdisc->flags & TCQ_F_NOLOCK) { spin_unlock(&qdisc->seqlock);
if (unlikely(test_bit(__QDISC_STATE_MISSED, &qdisc->state))) { clear_bit(__QDISC_STATE_MISSED, &qdisc->state); if (!(q->flags & TCQ_F_ONETXQUEUE) || !netif_xmit_frozen_or_stopped(q->dev_queue)) __netif_schedule(qdisc); }} }
For the strange non-ONETXQUEUE case we'd have an occasional unnecessary net_tx_action, but no infinite loop possible.
- clearing the STATE_MISSED for netif_xmit_frozen_or_stopped() case as this patch does, and protect the __netif_schedule() with q->seqlock for netif_tx_wake_queue(), which might bring unnecessary overhead for non-stopped queue case
Any better idea?
Or check the netif_xmit_frozen_or_stopped() again after clearing STATE_MISSED, like below:
if (netif_xmit_frozen_or_stopped(txq)) { clear_bit(__QDISC_STATE_MISSED, &q->state);
/* Make sure the below netif_xmit_frozen_or_stopped()
- checking happens after clearing STATE_MISSED.
*/ smp_mb__after_atomic();
/* Checking netif_xmit_frozen_or_stopped() again to
- make sure __QDISC_STATE_MISSED is set if the
- __QDISC_STATE_MISSED set by netif_tx_wake_queue()'s
- rescheduling of net_tx_action() is cleared by the
- above clear_bit().
*/ if (!netif_xmit_frozen_or_stopped(txq)) set_bit(__QDISC_STATE_MISSED, &q->state);
}
It is kind of ugly, but it does seem to fix the above data race too. And it seems like a common pattern to deal with the concurrency between xmit and NAPI polling, as below:
https://elixir.bootlin.com/linux/v5.12-rc2/source/drivers/net/ethernet/hisil...
This is indeed the idiomatic way of dealing with Tx queue stopping race, but it's a bit of code to sprinkle around. My vote would be option 1.
I had done some performance testing to see which is better, tested using pktgen and dummy netdev with pfifo_fast qdisc attached:
unit: Mpps
threads V6 V6 + option 1 V6 + option 3 1 2.60 2.54 2.60 2 3.86 3.84 3.84 4 5.56 5.50 5.51 8 2.79 2.77 2.77 16 2.23 2.24 2.22
So it seems the netif_xmit_frozen_or_stopped checking overhead for non-stopped queue is noticable for 1 pktgen thread.
And the performance increase for V6 + option 1 with 16 pktgen threads is because of "clear_bit(__QDISC_STATE_MISSED, &qdisc->state)" at the end of qdisc_run_end(), which may avoid the another round of dequeuing in the pfifo_fast_dequeue(). And adding the "clear_bit(__QDISC_STATE_MISSED, &qdisc->state)" for V6 + option 3, the data for 16 pktgen thread also go up to 2.24Mpps.
So it seems V6 + option 3 with "clear_bit(__QDISC_STATE_MISSED, &qdisc->state)" at the end of qdisc_run_end() is better?
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