Report RCU QS for busy network kthreads

This changeset fixes a common problem for busy networking kthreads. These threads, e.g. NAPI threads, typically will do:

* polling a batch of packets
* if there are more work, call cond_resched to allow scheduling
* continue to poll more packets when rx queue is not empty

We observed this being a problem in production, since it can block RCU tasks from making progress under heavy load. Investigation indicates that just calling cond_resched is insufficient for RCU tasks to reach quiescent states. This at least affects NAPI threads, napi_busy_loop, and also cpumap kthread for now.

By reporting RCU QSes in these kthreads periodically before cond_resched, the blocked RCU waiters can correctly progress. Instead of just reporting QS for RCU tasks, these code share the same concern as noted in the commit d28139c4e967 ("rcu: Apply RCU-bh QSes to RCU-sched and RCU-preempt when safe"). So report a consolidated QS for safety.

It is worth noting that, although this problem is reproducible in napi_busy_loop, it only shows up when setting the polling interval to as high as 2ms, which is far larger than recommended 50us-100us in the documentation. So napi_busy_loop is left untouched.

V2: https://lore.kernel.org/bpf/[email protected]/
V1: https://lore.kernel.org/lkml/[email protected]/#t

changes since v2:
* created a helper in rcu header to abstract the behavior
* fixed cpumap kthread in addition

changes since v1:
* disable preemption first as Paul McKenney suggested

Yan Zhai (3):
rcu: add a helper to report consolidated flavor QS
net: report RCU QS on threaded NAPI repolling
bpf: report RCU QS in cpumap kthread

 include/linux/rcupdate.h | 23 +++++++++++++++++++++++
 kernel/bpf/cpumap.c      |  2 ++
 net/core/dev.c           |  3 +++
 3 files changed, 28 insertions(+)

2.30.2

There are several scenario in network processing that can run extensively under heavy traffic. In such situation, RCU synchronization might not observe desired quiescent states for indefinitely long period. Create a helper to safely raise the desired RCU quiescent states for such scenario.

---
 include/linux/rcupdate.h | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

index 0746b1b0b663..e91ae38c33e3 100644
@@ -247,6 +247,29 @@ do { \
 	cond_resched(); \
 } while (0)
 
+/**
+ * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states
+ *
+ * This helper is for network processing in non-RT kernels, where there could
+ * be busy polling threads that block RCU synchronization indefinitely.  In
+ * such context, simply calling cond_resched is insufficient, so give it a
+ * stronger push to eliminate potential blockage of all RCU types.
+ *
+ * NOTE: unless absolutely sure, this helper should in general be called
+ * outside of bh lock section to avoid reporting a surprising QS to updaters,
+ * who could be expecting RCU read critical section to end at local_bh_enable().
+ */
+#define rcu_softirq_qs_periodic(old_ts) \
+do { \
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \
+	    time_after(jiffies, (old_ts) + HZ / 10)) { \
+		preempt_disable(); \
+		rcu_softirq_qs(); \
+		preempt_enable(); \
+		(old_ts) = jiffies; \
+	} \
+} while (0)
+
 /*
  * Infrastructure to implement the synchronize_() primitives in
  * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.

2.30.2

TIL, thanks. Let me send v4 to amend the text.

Yan

NAPI threads can keep polling packets under load. Currently it is only calling cond_resched() before repolling, but it is not sufficient to clear out the holdout of RCU tasks, which prevent BPF tracing programs from detaching for long period. This can be reproduced easily with following set up:

ip netns add test1
ip netns add test2

ip -n test1 link add veth1 type veth peer name veth2 netns test2

ip -n test1 link set veth1 up
ip -n test1 link set lo up
ip -n test2 link set veth2 up
ip -n test2 link set lo up

ip -n test1 addr add 192.168.1.2/31 dev veth1
ip -n test1 addr add 1.1.1.1/32 dev lo
ip -n test2 addr add 192.168.1.3/31 dev veth2
ip -n test2 addr add 2.2.2.2/31 dev lo

ip -n test1 route add default via 192.168.1.3
ip -n test2 route add default via 192.168.1.2

for i in seq 10 210; do
for j in seq 10 210; do

    ip netns exec test2 iptables -I INPUT -s 3.3.$i.$j -p udp --dport 5201

done
done

ip netns exec test2 ethtool -K veth2 gro on
ip netns exec test2 bash -c 'echo 1 > /sys/class/net/veth2/threaded' ip netns exec test1 ethtool -K veth1 tso off

Then run an iperf3 client/server and a bpftrace script can trigger it:

ip netns exec test2 iperf3 -s -B 2.2.2.2 >/dev/null&
ip netns exec test1 iperf3 -c 2.2.2.2 -B 1.1.1.1 -u -l 1500 -b 3g -t 100 >/dev/null& bpftrace -e 'kfunc:__napi_poll{@=count();} interval:s:1{exit();}'

Report RCU quiescent states periodically will resolve the issue.

---
v2->v3: abstracted the work into a RCU helper
v1->v2: moved rcu_softirq_qs out from bh critical section, and only raise it after a second of repolling. Added some brief perf test result.

v2: https://lore.kernel.org/bpf/[email protected]/
v1: https://lore.kernel.org/lkml/[email protected]/#t

---
 net/core/dev.c | 3 +++
 1 file changed, 3 insertions(+)

index 76e6438f4858..6b7fc42d4b3e 100644
@@ -6708,6 +6708,8 @@ static int napi_threaded_poll(void *data)
 	void *have;
 
 	while (!napi_thread_wait(napi)) {
+		unsigned long last_qs = jiffies;
+
 		for (;;) {
 			bool repoll = false;
 

@@ -6732,6 +6734,7 @@ static int napi_threaded_poll(void *data)
 			if (!repoll)
 				break;
 
+			rcu_softirq_qs_periodic(last_qs);
 			cond_resched();
 		}
 	}

2.30.2

When there are heavy load, cpumap kernel threads can be busy polling packets from redirect queues and block out RCU tasks from reaching quiescent states. It is insufficient to just call cond_resched() in such context. Periodically raise a consolidated RCU QS before cond_resched fixes the problem.

---
 kernel/bpf/cpumap.c | 3 +++
 1 file changed, 3 insertions(+)

index ef82ffc90cbe..8f1d390bcbde 100644
@@ -262,6 +262,7 @@ static int cpu_map_bpf_prog_run(struct bpf_cpu_map_entry *rcpu, void **frames,
 static int cpu_map_kthread_run(void *data)
 {
 	struct bpf_cpu_map_entry *rcpu = data;
+	unsigned long last_qs = jiffies;
 
 	complete(&rcpu->kthread_running);
 	set_current_state(TASK_INTERRUPTIBLE);

@@ -287,10 +288,12 @@ static int cpu_map_kthread_run(void *data)
 			if (__ptr_ring_empty(rcpu->queue)) {
 				schedule();
 				sched = 1;
+				last_qs = jiffies;
 			} else {
 				__set_current_state(TASK_RUNNING);
 			}
 		} else {
+			rcu_softirq_qs_periodic(last_qs);
 			sched = cond_resched();
 		}
 

2.30.2

For the series: