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@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>
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
