Re: [PATCH net v3] net: sched: fix packet stuck problem for lockless qdisc

[Juergen Gross]

On 25.03.21 04:13, Yunsheng Lin wrote:
> Lockless qdisc has below concurrent problem:
>      cpu0                 cpu1
>       .                     .
> q->enqueue                 .
>       .                     .
> qdisc_run_begin()          .
>       .                     .
> dequeue_skb()              .
>       .                     .
> sch_direct_xmit()          .
>       .                     .
>       .                q->enqueue
>       .             qdisc_run_begin()
>       .            return and do nothing
>       .                     .
> qdisc_run_end()            .
>
> cpu1 enqueue a skb without calling __qdisc_run() because cpu0
> has not released the lock yet and spin_trylock() return false
> for cpu1 in qdisc_run_begin(), and cpu0 do not see the skb
> enqueued by cpu1 when calling dequeue_skb() because cpu1 may
> enqueue the skb after cpu0 calling dequeue_skb() and before
> cpu0 calling qdisc_run_end().
>
> Lockless qdisc has below another concurrent problem when
> tx_action is involved:
>
> cpu0(serving tx_action)     cpu1             cpu2
>            .                   .                .
>            .              q->enqueue            .
>            .            qdisc_run_begin()       .
>            .              dequeue_skb()         .
>            .                   .            q->enqueue
>            .                   .                .
>            .             sch_direct_xmit()      .
>            .                   .         qdisc_run_begin()
>            .                   .       return and do nothing
>            .                   .                .
>   clear __QDISC_STATE_SCHED    .                .
>   qdisc_run_begin()            .                .
>   return and do nothing        .                .
>            .                   .                .
>            .            qdisc_run_end()         .
>
> This patch fixes the above data race by:
> 1. Get the flag before doing spin_trylock().
> 2. If the first spin_trylock() return false and the flag is not
>     set before the first spin_trylock(), Set the flag and retry
>     another spin_trylock() in case other CPU may not see the new
>     flag after it releases the lock.
> 3. reschedule if the flags is set after the lock is released
>     at the end of qdisc_run_end().
>
> For tx_action case, the flags is also set when cpu1 is at the
> end if qdisc_run_end(), so tx_action will be rescheduled
> again to dequeue the skb enqueued by cpu2.
>
> Only clear the flag before retrying a dequeuing when dequeuing
> returns NULL in order to reduce the overhead of the above double
> spin_trylock() and __netif_schedule() calling.
>
> The performance impact of this patch, tested using pktgen and
> dummy netdev with pfifo_fast qdisc attached:
>
>   threads  without+this_patch   with+this_patch      delta
>      1        2.61Mpps            2.60Mpps           -0.3%
>      2        3.97Mpps            3.82Mpps           -3.7%
>      4        5.62Mpps            5.59Mpps           -0.5%
>      8        2.78Mpps            2.77Mpps           -0.3%
>     16        2.22Mpps            2.22Mpps           -0.0%
>
> Fixes: 6b3ba9146fe6 ("net: sched: allow qdiscs to handle locking")
> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>

I have a setup which is able to reproduce the issue quite reliably:

In a Xen guest I'm mounting 8 NFS shares and run sysbench fileio on
each of them. The average latency reported by sysbench is well below
1 msec, but at least once per hour I get latencies in the minute
range.

With this patch I don't see these high latencies any longer (test
is running for more than 20 hours now).

So you can add my:

Tested-by: Juergen Gross <jgross@xxxxxxxx>


Juergen

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