On Fri, Apr 16, 2021 at 09:16:48AM +0800, 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:
- Test STATE_MISSED before doing spin_trylock().
- If the first spin_trylock() return false and STATE_MISSED is not set before the first spin_trylock(), Set STATE_MISSED and retry another spin_trylock() in case other CPU may not see STATE_MISSED after it releases the lock.
- reschedule if STATE_MISSED is set after the lock is released at the end of qdisc_run_end().
For tx_action case, STATE_MISSED 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.
Clear STATE_MISSED 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 Tested-by: Juergen Gross jgross@suse.com
V4: Change STATE_NEED_RESCHEDULE to STATE_MISSED mirroring NAPI's NAPIF_STATE_MISSED, and add Juergen's "Tested-by" tag for there is only renaming and typo fixing between V4 and V3. V3: Fix a compile error and a few comment typo, remove the __QDISC_STATE_DEACTIVATED checking, and update the performance data. V2: Avoid the overhead of fixing the data race as much as possible.
As pointed out in the discussion on v3, this patch may result in significantly higher CPU consumption with multiple threads competing on a saturated outgoing device. I missed this submission so that I haven't checked it yet but given the description of v3->v4 changes above, it's quite likely that it suffers from the same problem.
Michal