Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Wed, 28 Feb 2024 12:50:53 +0100

"Paul E. McKenney" <[email-masked]> writes:

> On Tue, Feb 27, 2024 at 05:44:17PM +0100, Eric Dumazet wrote:
>> On Tue, Feb 27, 2024 at 4:44 PM Yan Zhai <[email-masked]> wrote:
>> >
>> > 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 <[email-masked]>
>> > ---
>> >  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);
>
> Please put a comment here stating that RCU readers cannot cross
> this point.
>
> I need to add lockdep to rcu_softirq_qs() to catch placing this in an
> RCU read-side critical section.  And a header comment noting that from
> an RCU perspective, it acts as a momentary enabling of preemption.

OK, so one question here: for XDP, we're basically treating
local_bh_disable/enable() as the RCU critical section, cf the discussion
we had a few years ago that led to this being documented[0]. So why is
it OK to have the rcu_softirq_qs() inside the bh disable/enable pair,
but not inside an rcu_read_lock() section?

Also, looking at the patch in question:

>> > +                       if (!IS_ENABLED(CONFIG_PREEMPT_RT))
>> > +                               rcu_softirq_qs();
>> > +
>> >                         local_bh_enable();

Why does that local_bh_enable() not accomplish the same thing as the qs?

-Toke

[0] https://lore.kernel.org/bpf/[email-masked]/