On Mon, Apr 19, 2021 at 10:04:27AM +0800, Yunsheng Lin wrote:
I tried this patch o top of 5.12-rc7 with real devices. I used two machines with 10Gb/s Intel ixgbe NICs, sender has 16 CPUs (2 8-core CPUs with HT disabled) and 16 Rx/Tx queues, receiver has 48 CPUs (2 12-core CPUs with HT enabled) and 48 Rx/Tx queues. With multiple TCP streams on a saturated ethernet, the CPU consumption grows quite a lot:
threads unpatched 5.12-rc7 5.12-rc7 + v3 1 25.6% 30.6% 8 73.1% 241.4% 128 132.2% 1012.0%(The values are normalized to one core, i.e. 100% corresponds to one fully used logical CPU.) I didn't perform a full statistical evaluation but the growth is way beyond any statistical fluctuation with one exception: 8-thread test of patched kernel showed values from 155.5% to 311.4%. Closer look shows that most of the CPU time was spent in softirq and running top in parallel with the test confirms that there are multiple ksofirqd threads running at 100% CPU. I had similar problem with earlier versions of my patch (work in progress, I still need to check some corner cases and write commit message explaining the logic)
Great, if there is a better idea, maybe share the core idea first so that we both can work on the that?
I'm not sure if it's really better but to minimize the false positives and unnecessary calls to __netif_schedule(), I replaced q->seqlock with an atomic combination of a "running" flag (which corresponds to current seqlock being locked) and a "drainers" count (number of other threads going to clean up the qdisc queue). This way we could keep track of them and get reliable information if another thread is going to run a cleanup after we leave the qdisc_run() critical section (so that there is no need to schedule).
The biggest problem IMHO is that the loop in __qdisc_run() may finish without rescheduling not only when the qdisc queue is empty but also when the corresponding device Tx queue is stopped which devices tend to do whenever they cannot send any more packets out. Thus whenever __QDISC_STATE_NEED_RESCHEDULE is set with device queue stopped or frozen, we keep rescheduling the queue cleanup without any chance to progress or clear __QDISC_STATE_NEED_RESCHEDULE. For this to happen, all we need is another thready to fail the first spin_trylock() while device queue is stopped and qdisc queue not empty.
Yes, We could just return false before doing the second spin_trylock() if the netdev queue corresponding qdisc is stopped, and when the netdev queue is restarted, __netif_schedule() is called again, see netif_tx_wake_queue().
Maybe add a sch_qdisc_stopped() function and do the testting in qdisc_run_begin:
if (dont_retry || sch_qdisc_stopped()) return false;
bool sch_qdisc_stopped(struct Qdisc *q) { const struct netdev_queue *txq = q->dev_queue;
if (!netif_xmit_frozen_or_stopped(txq)) return true;
reture false; }
At least for qdisc with TCQ_F_ONETXQUEUE flags set is doable?
Either this or you can do the check in qdisc_run_end() - when the device queue is stopped or frozen, there is no need to schedule as we know it's going to be done when the flag is cleared again (and we cannot do anything until then anyway).
Another part of the problem may be that to avoid the race, the logic is too pessimistic: consider e.g. (dotted lines show "barriers" where ordering is important):
CPU A CPU B spin_trylock() succeeds pfifo_fast_enqueue() .................................................................. skb_array empty, exit loop first spin_trylock() fails set __QDISC_STATE_NEED_RESCHEDULE second spin_trylock() fails .................................................................. spin_unlock() call __netif_schedule()When we switch the order of spin_lock() on CPU A and second spin_trylock() on CPU B (but keep setting __QDISC_STATE_NEED_RESCHEDULE before CPU A tests it), we end up scheduling a queue cleanup even if there is already one running. And either of these is quite realistic.
But I am not sure it is a good thing or bad thing when the above happen, because it may be able to enable the tx batching?
In either of the two scenarios, we call __netif_schedule() to schedule a cleanup which does not do anything useful. In first, the qdics queue is empty so that either the scheduled cleanup finds it empty or there will be some new packets which would have their own cleanup. In second, scheduling a cleanup will not prevent the other thread from doing its own cleanup it already started.
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c index 44991ea..4953430 100644 --- a/net/sched/sch_generic.c +++ b/net/sched/sch_generic.c @@ -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 +654,16 @@ 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_and_clear_bit(__QDISC_STATE_NEED_RESCHEDULE,&qdisc->state)) {/* do another enqueuing after clearing the flag to* avoid calling __netif_schedule().*/smp_mb__after_atomic();need_retry = false;goto retry;Does the retry really provide significant improvement? IIUC it only helps if all of enqueue, first spin_trylock() failure and setting the __QDISC_STATE_NEED_RESCHEDULE flag happen between us finding the skb_array empty and checking the flag. If enqueue happens before we check the array (and flag is set before the check), the retry is useless. If the flag is set after we check it, we don't catch it (no matter if the enqueue happened before or after we found skb_array empty).
In odrder to avoid doing the "set_bit(__QDISC_STATE_MISSED, &qdisc->state)" as much as possible, the __QDISC_STATE_NEED_RESCHEDULE need to be set as as much as possible, so only clear __QDISC_STATE_NEED_RESCHEDULE when the queue is empty.
This is what I'm worried about. We are trying to address a race condition which is extremely rare so we should avoid adding too much overhead to the normal use.
And it has about 5% performance improvement.
OK then. It thought that it would do an unnecessary dequeue retry much more often than prevent an unnecessary __netif_schedule() but I did not run any special benchmark to check.
Michal