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:
- Get the flag before doing spin_trylock().
- 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.
- 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
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:
- Get the flag before doing spin_trylock().
- 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.
- 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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Juergen Gross -
Yunsheng Lin