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net: sched: fix packet stuck problem for lockless qdisc

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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. If the first spin_trylock() return false and STATE_MISSED is
   not set, set STATE_MISSED and retry another spin_trylock() in
   case other CPU may not see STATE_MISSED after it releases the
   lock.
2. 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 second
spin_trylock() and __netif_schedule() calling.

Also clear the STATE_MISSED before calling __netif_schedule()
at the end of qdisc_run_end() to avoid doing another round of
dequeuing in the pfifo_fast_dequeue().

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")
Acked-by: Jakub Kicinski <kuba@kernel.org>
Tested-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Yunsheng Lin 2021-05-14 11:16:59 +08:00 committed by David S. Miller
parent 974271e5ed
commit a90c57f2ce
2 changed files with 53 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

35
include/net/sch_generic.h
View File

@ -36,6 +36,7 @@ struct qdisc_rate_table {
enum qdisc_state_t {
__QDISC_STATE_SCHED,
__QDISC_STATE_DEACTIVATED,
__QDISC_STATE_MISSED,
};
struct qdisc_size_table {
@ -159,8 +160,33 @@ 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))
goto nolock_empty;
/* If the MISSED flag is set, it means other thread has
* set the MISSED flag before second spin_trylock(), so
* we can return false here to avoid multi cpus doing
* the set_bit() and second spin_trylock() concurrently.
*/
if (test_bit(__QDISC_STATE_MISSED, &qdisc->state))
return false;
/* Set the MISSED flag before the second spin_trylock(),
* if the second spin_trylock() return false, it means
* other cpu holding the lock will do dequeuing for us
* or it will see the MISSED flag set after releasing
* lock and reschedule the net_tx_action() to do the
* dequeuing.
*/
set_bit(__QDISC_STATE_MISSED, &qdisc->state);
/* Retry again in case other CPU may not see the new flag
* after it releases the lock at the end of qdisc_run_end().
*/
if (!spin_trylock(&qdisc->seqlock))
return false;
nolock_empty:
WRITE_ONCE(qdisc->empty, false);
} else if (qdisc_is_running(qdisc)) {
return false;
@ -176,8 +202,15 @@ 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_MISSED,
&qdisc->state))) {
clear_bit(__QDISC_STATE_MISSED, &qdisc->state);
__netif_schedule(qdisc);
}
}
}
static inline bool qdisc_may_bulk(const struct Qdisc *qdisc)

19
net/sched/sch_generic.c
View File

@ -640,8 +640,10 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
{
struct pfifo_fast_priv *priv = qdisc_priv(qdisc);
struct sk_buff *skb = NULL;
bool need_retry = true;
int band;
retry:
for (band = 0; band < PFIFO_FAST_BANDS && !skb; band++) {
struct skb_array *q = band2list(priv, band);
@ -652,6 +654,23 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
}
if (likely(skb)) {
qdisc_update_stats_at_dequeue(qdisc, skb);
} else if (need_retry &&
test_bit(__QDISC_STATE_MISSED, &qdisc->state)) {
/* Delay clearing the STATE_MISSED here to reduce
* the overhead of the second spin_trylock() in
* qdisc_run_begin() and __netif_schedule() calling
* in qdisc_run_end().
*/
clear_bit(__QDISC_STATE_MISSED, &qdisc->state);
/* Make sure dequeuing happens after clearing
* STATE_MISSED.
*/
smp_mb__after_atomic();
need_retry = false;
goto retry;
} else {
WRITE_ONCE(qdisc->empty, true);
}