netdev - Re: [net PATCH v2] net: sched, fix OOO packets with pfifo

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Date:   Mon, 26 Mar 2018 12:36:43 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     john.fastabend@...il.com
Cc:     eric.dumazet@...il.com, xiyou.wangcong@...il.com, jiri@...nulli.us,
        netdev@...r.kernel.org
Subject: Re: [net PATCH v2] net: sched, fix OOO packets with pfifo_fast

From: John Fastabend <john.fastabend@...il.com>
Date: Sat, 24 Mar 2018 22:25:06 -0700

> After the qdisc lock was dropped in pfifo_fast we allow multiple
> enqueue threads and dequeue threads to run in parallel. On the
> enqueue side the skb bit ooo_okay is used to ensure all related
> skbs are enqueued in-order. On the dequeue side though there is
> no similar logic. What we observe is with fewer queues than CPUs
> it is possible to re-order packets when two instances of
> __qdisc_run() are running in parallel. Each thread will dequeue
> a skb and then whichever thread calls the ndo op first will
> be sent on the wire. This doesn't typically happen because
> qdisc_run() is usually triggered by the same core that did the
> enqueue. However, drivers will trigger __netif_schedule()
> when queues are transitioning from stopped to awake using the
> netif_tx_wake_* APIs. When this happens netif_schedule() calls
> qdisc_run() on the same CPU that did the netif_tx_wake_* which
> is usually done in the interrupt completion context. This CPU
> is selected with the irq affinity which is unrelated to the
> enqueue operations.
> 
> To resolve this we add a RUNNING bit to the qdisc to ensure
> only a single dequeue per qdisc is running. Enqueue and dequeue
> operations can still run in parallel and also on multi queue
> NICs we can still have a dequeue in-flight per qdisc, which
> is typically per CPU.
> 
> Fixes: c5ad119fb6c0 ("net: sched: pfifo_fast use skb_array")
> Reported-by: Jakob Unterwurzacher <jakob.unterwurzacher@...obroma-systems.com>
> Signed-off-by: John Fastabend <john.fastabend@...il.com>

Applied, thanks John.