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

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2021/4/9 13:31, Juergen Gross wrote:
> 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@huawei.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@suse.com>

Hi, Juergen

Thanks for the testing.

With the simulated test case suggested by Michal, I still has some
potential issue to debug, hopefully will send out new version in
this week.

Also, is it possible to run your testcase any longer? I think "72 hours"
would be enough to testify that it fixes the problem completely:)



> 
> 
> Juergen