netdev - Re: [PATCH net] net: sched: fix reordering issues

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Date:   Fri, 06 Sep 2019 15:13:36 +0200 (CEST)
From:   David Miller <davem@...emloft.net>
To:     edumazet@...gle.com
Cc:     netdev@...r.kernel.org, eric.dumazet@...il.com,
        john.fastabend@...il.com
Subject: Re: [PATCH net] net: sched: fix reordering issues

From: Eric Dumazet <edumazet@...gle.com>
Date: Thu,  5 Sep 2019 05:20:22 -0700

> Whenever MQ is not used on a multiqueue device, we experience
> serious reordering problems. Bisection found the cited
> commit.
> 
> The issue can be described this way :
> 
> - A single qdisc hierarchy is shared by all transmit queues.
>   (eg : tc qdisc replace dev eth0 root fq_codel)
> 
> - When/if try_bulk_dequeue_skb_slow() dequeues a packet targetting
>   a different transmit queue than the one used to build a packet train,
>   we stop building the current list and save the 'bad' skb (P1) in a
>   special queue. (bad_txq)
> 
> - When dequeue_skb() calls qdisc_dequeue_skb_bad_txq() and finds this
>   skb (P1), it checks if the associated transmit queues is still in frozen
>   state. If the queue is still blocked (by BQL or NIC tx ring full),
>   we leave the skb in bad_txq and return NULL.
> 
> - dequeue_skb() calls q->dequeue() to get another packet (P2)
> 
>   The other packet can target the problematic queue (that we found
>   in frozen state for the bad_txq packet), but another cpu just ran
>   TX completion and made room in the txq that is now ready to accept
>   new packets.
> 
> - Packet P2 is sent while P1 is still held in bad_txq, P1 might be sent
>   at next round. In practice P2 is the lead of a big packet train
>   (P2,P3,P4 ...) filling the BQL budget and delaying P1 by many packets :/
> 
> To solve this problem, we have to block the dequeue process as long
> as the first packet in bad_txq can not be sent. Reordering issues
> disappear and no side effects have been seen.
> 
> Fixes: a53851e2c321 ("net: sched: explicit locking in gso_cpu fallback")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied and queued up for -stable, thanks Eric.