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

[Juergen Gross <jgross@suse.com>]

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On 12.04.21 03:04, Yunsheng Lin wrote:
> 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:)

This should be possible, yes.

I'm using the setup to catch another bug which is showing up every few
days. I don't see a reason why I shouldn't be able to add your patch
and verify it in parallel.


Juergen

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