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

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Feb 27, 2024 at 05:44:17PM +0100, Eric Dumazet wrote:
> On Tue, Feb 27, 2024 at 4:44 PM Yan Zhai <yan@cloudflare.com> 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 <yan@cloudflare.com>
> > ---
> >  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.

> > +                       if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> > +                               rcu_softirq_qs();
> > +
> >                         local_bh_enable();
> >
> >                         if (!repoll)
> > --
> > 2.30.2
> >
> 
> Hmm....
> Why napi_busy_loop() does not have a similar problem ?
> 
> It is unclear why rcu_all_qs() in __cond_resched() is guarded by
> 
> #ifndef CONFIG_PREEMPT_RCU
>      rcu_all_qs();
> #endif

The theory is that PREEMPT_RCU kernels have preemption, and get their
quiescent states that way.

The more recent practice involves things like PREEMPT_DYNAMIC and maybe
soon PREEMPT_AUTO, which might require adjustments, so thank you for
pointing this out!

Back on the patch, my main other concern is that someone somewhere might
be using something like synchronize_rcu() to wait for all in-progress
softirq handlers to complete.  But I don't know of such a thing, and if
there is, there are workarounds, including synchronize_rcu_tasks().

So something to be aware of, not (as far as I know) something to block
this commit.

With the added comment:

Acked-by: Paul E. McKenney <paulmck@kernel.org>

							Thanx, Paul