Re: [PATCH net] net: sched: fix packet stuck problem for lockless qdisc
From: Yunsheng Lin
Date: Sun Mar 21 2021 - 21:32:58 EST
On 2021/3/20 3:40, Cong Wang wrote:
> On Wed, Mar 17, 2021 at 11:52 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> 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 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_begin() .
>>
>> This patch fixes the above data race by:
>> 1. Set a flag after spin_trylock() return false.
>> 2. Retry a spin_trylock() in case other CPU may not see the
>> new flag after it releases the lock.
>> 3. reschedule if the flag 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_begin(), so tx_action will be rescheduled
>> again to dequeue the skb enqueued by cpu2.
>>
>> Also clear the flag before dequeuing in order to reduce the
>> overhead of the above process, and aviod doing the heavy
>> test_and_clear_bit() at the end of qdisc_run_end().
>>
>> Fixes: 6b3ba9146fe6 ("net: sched: allow qdiscs to handle locking")
>> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
>> ---
>> For those who has not been following the qdsic scheduling
>> discussion, there is packet stuck problem for lockless qdisc,
>> see [1], and I has done some cleanup and added some enhanced
>> features too, see [2] [3].
>> While I was doing the optimization for lockless qdisc, it
>> accurred to me that these optimization is useless if there is
>> still basic bug in lockless qdisc, even the bug is not easily
>> reproducible. So look through [1] again, I found that the data
>> race for tx action mentioned by Michael, and thought deep about
>> it and came up with this patch trying to fix it.
>>
>> So I am really appreciated some who still has the reproducer
>> can try this patch and report back.
>>
>> 1. https://lore.kernel.org/netdev/d102074f-7489-e35a-98cf-e2cad7efd8a2@xxxxxxxxxxxxx/t/#ma7013a79b8c4d8e7c49015c724e481e6d5325b32
>> 2. https://patchwork.kernel.org/project/netdevbpf/patch/1615777818-13969-1-git-send-email-linyunsheng@xxxxxxxxxx/
>> 3. https://patchwork.kernel.org/project/netdevbpf/patch/1615800610-34700-1-git-send-email-linyunsheng@xxxxxxxxxx/
>> ---
>> include/net/sch_generic.h | 23 ++++++++++++++++++++---
>> net/sched/sch_generic.c | 1 +
>> 2 files changed, 21 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
>> index f7a6e14..4220eab 100644
>> --- a/include/net/sch_generic.h
>> +++ b/include/net/sch_generic.h
>> @@ -36,6 +36,7 @@ struct qdisc_rate_table {
>> enum qdisc_state_t {
>> __QDISC_STATE_SCHED,
>> __QDISC_STATE_DEACTIVATED,
>> + __QDISC_STATE_NEED_RESCHEDULE,
>> };
>>
>> struct qdisc_size_table {
>> @@ -159,8 +160,17 @@ static inline bool qdisc_is_empty(const struct Qdisc *qdisc)
>> static inline bool qdisc_run_begin(struct Qdisc *qdisc)
>> {
>> if (qdisc->flags & TCQ_F_NOLOCK) {
>> - if (!spin_trylock(&qdisc->seqlock))
>> - return false;
>> + if (!spin_trylock(&qdisc->seqlock)) {
>> + set_bit(__QDISC_STATE_NEED_RESCHEDULE,
>> + &qdisc->state);
>
> Why do we need another bit? I mean why not just call __netif_schedule()?
I think that was your proposal in [1], the only difference is that
it also handle the tx_action case when __netif_schedule() is called
here.
So yes, it can also fix the two data race described in this patch, but
with a bigger performance degradation, quoting performance data in the
[1]:
pktgen threads vanilla patched delta
nr kpps kpps %
1 3240 3240 0
2 3910 2710 -30.5
4 5140 4920 -4
performance data with this patch:
threads vanilla vanilla+this_patch delta
1 2.6Mpps 2.5Mpps -3%
2 3.9Mpps 3.6Mpps -7%
4 5.6Mpps 4.7Mpps -16%
So the performance is why I does not call __netif_schedule() directly
here.
1. https://lore.kernel.org/netdev/d102074f-7489-e35a-98cf-e2cad7efd8a2@xxxxxxxxxxxxx/t/#md927651488ce4d226f6279aad6699b4bee4674a3
>
>> +
>> + /* Retry again in case other CPU may not see the
>> + * new flags after it releases the lock at the
>> + * end of qdisc_run_end().
>> + */
>> + if (!spin_trylock(&qdisc->seqlock))
>> + return false;
>> + }
>> WRITE_ONCE(qdisc->empty, false);
>> } else if (qdisc_is_running(qdisc)) {
>> return false;
>> @@ -176,8 +186,15 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
>> static inline void qdisc_run_end(struct Qdisc *qdisc)
>> {
>> write_seqcount_end(&qdisc->running);
>> - if (qdisc->flags & TCQ_F_NOLOCK)
>> + if (qdisc->flags & TCQ_F_NOLOCK) {
>> spin_unlock(&qdisc->seqlock);
>> +
>> + if (unlikely(test_bit(__QDISC_STATE_NEED_RESCHEDULE,
>> + &qdisc->state) &&
>> + !test_bit(__QDISC_STATE_DEACTIVATED,
>> + &qdisc->state)))
>
> Testing two bits one by one is not atomic...
For non-tx_action case, actually it is atomic because the above
two bits testing is within the rcu protection, and qdisc reset
will do a synchronize_net() after setting __QDISC_STATE_DEACTIVATED.
For tx_action case, I think we need a rcu protection explicitly in
net_tx_action() too, at least for PREEMPT_RCU:
https://stackoverflow.com/questions/21287932/is-it-necessary-invoke-rcu-read-lock-in-softirq-context
>
>
>> + __netif_schedule(qdisc);
>> + }
>> }
>>
>> static inline bool qdisc_may_bulk(const struct Qdisc *qdisc)
>> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
>> index 44991ea..25d75d8 100644
>> --- a/net/sched/sch_generic.c
>> +++ b/net/sched/sch_generic.c
>> @@ -205,6 +205,7 @@ static struct sk_buff *dequeue_skb(struct Qdisc *q, bool *validate,
>> const struct netdev_queue *txq = q->dev_queue;
>> struct sk_buff *skb = NULL;
>>
>> + clear_bit(__QDISC_STATE_NEED_RESCHEDULE, &q->state);
>> *packets = 1;
>> if (unlikely(!skb_queue_empty(&q->gso_skb))) {
>> spinlock_t *lock = NULL;
>> --
>> 2.7.4
>>
>
> .
>