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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Jiri Pirko , Simon Horman , Daniel Borkmann , Lorenzo Bianconi , Coco Li , Wei Wang , Alexander Duyck , Hannes Frederic Sowa , linux-kernel@vger.kernel.org, rcu@vger.kernel.org, bpf@vger.kernel.org, kernel-team@cloudflare.com Subject: [PATCH] net: raise RCU qs after each threaded NAPI poll Message-ID: Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Disposition: inline We noticed task RCUs being blocked when threaded NAPIs are very busy in production: detaching any BPF tracing programs, i.e. removing a ftrace trampoline, will simply block for very long in rcu_tasks_wait_gp. This ranges from hundreds of seconds to even an hour, severely harming any observability tools that rely on BPF tracing programs. It can be easily reproduced locally with following setup: 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();}' Above reproduce for net-next kernel with following RCU and preempt configuraitons: # RCU Subsystem CONFIG_TREE_RCU=y CONFIG_PREEMPT_RCU=y # CONFIG_RCU_EXPERT is not set CONFIG_SRCU=y CONFIG_TREE_SRCU=y CONFIG_TASKS_RCU_GENERIC=y CONFIG_TASKS_RCU=y CONFIG_TASKS_RUDE_RCU=y CONFIG_TASKS_TRACE_RCU=y CONFIG_RCU_STALL_COMMON=y CONFIG_RCU_NEED_SEGCBLIST=y # end of RCU Subsystem # RCU Debugging # CONFIG_RCU_SCALE_TEST is not set # CONFIG_RCU_TORTURE_TEST is not set # CONFIG_RCU_REF_SCALE_TEST is not set CONFIG_RCU_CPU_STALL_TIMEOUT=21 CONFIG_RCU_EXP_CPU_STALL_TIMEOUT=0 # CONFIG_RCU_TRACE is not set # CONFIG_RCU_EQS_DEBUG is not set # end of RCU Debugging CONFIG_PREEMPT_BUILD=y # CONFIG_PREEMPT_NONE is not set CONFIG_PREEMPT_VOLUNTARY=y # CONFIG_PREEMPT is not set CONFIG_PREEMPT_COUNT=y CONFIG_PREEMPTION=y CONFIG_PREEMPT_DYNAMIC=y CONFIG_PREEMPT_RCU=y CONFIG_HAVE_PREEMPT_DYNAMIC=y CONFIG_HAVE_PREEMPT_DYNAMIC_CALL=y CONFIG_PREEMPT_NOTIFIERS=y # CONFIG_DEBUG_PREEMPT is not set # CONFIG_PREEMPT_TRACER is not set # CONFIG_PREEMPTIRQ_DELAY_TEST is not set An interesting observation is that, while tasks RCUs are blocked, related NAPI thread is still being scheduled (even across cores) regularly. Looking at the gp conditions, I am inclining to cond_resched after each __napi_poll being the problem: cond_resched enters the scheduler with PREEMPT bit, which does not account as a gp for tasks RCUs. Meanwhile, since the thread has been frequently resched, the normal scheduling point (no PREEMPT bit, accounted as a task RCU gp) seems to have very little chance to kick in. Given the nature of "busy polling" program, such NAPI thread won't have task->nvcsw or task->on_rq updated (other gp conditions), the result is that such NAPI thread is put on RCU holdouts list for indefinitely long time. This is simply fixed by mirroring the ksoftirqd behavior: after NAPI/softirq work, raise a RCU QS to help expedite the RCU period. No more blocking afterwards for the same setup. Fixes: 29863d41bb6e ("net: implement threaded-able napi poll loop support") Signed-off-by: Yan Zhai Acked-by: Paul E. McKenney Reviewed-by: Joel Fernandes (Google) Signed-off-by: Mark Rutland --- net/core/dev.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/net/core/dev.c b/net/core/dev.c index 275fd5259a4a..6e41263ff5d3 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -6773,6 +6773,10 @@ static int napi_threaded_poll(void *data) net_rps_action_and_irq_enable(sd); } skb_defer_free_flush(sd); + + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) + rcu_softirq_qs(); + local_bh_enable(); if (!repoll)