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

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Date:   Fri, 9 Apr 2021 07:31:03 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net,
        kuba@...nel.org
Cc:     olteanv@...il.com, ast@...nel.org, daniel@...earbox.net,
        andriin@...com, edumazet@...gle.com, weiwan@...gle.com,
        cong.wang@...edance.com, ap420073@...il.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxarm@...neuler.org, mkl@...gutronix.de,
        linux-can@...r.kernel.org, jhs@...atatu.com,
        xiyou.wangcong@...il.com, jiri@...nulli.us, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, bpf@...r.kernel.org,
        jonas.bonn@...rounds.com, pabeni@...hat.com, mzhivich@...mai.com,
        johunt@...mai.com, albcamus@...il.com, kehuan.feng@...il.com,
        a.fatoum@...gutronix.de, atenart@...nel.org,
        alexander.duyck@...il.com, Jiri Kosina <JKosina@...e.com>
Subject: Re: [PATCH net v3] net: sched: fix packet stuck problem for lockless
 qdisc

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@...wei.com>

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@...e.com>


Juergen

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