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From: Yan Zhai <yan@cloudflare.com>
Date: Wed, 28 Feb 2024 17:53:33 -0600
Message-ID: <CAO3-Pbq4Fybyhodv5-36U=-rgttkjxFj6cRvAGcapvE8pZyWSQ@mail.gmail.com>
Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll
To: Eric Dumazet <edumazet@google.com>
Cc: netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>, 
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>, Jiri Pirko <jiri@resnulli.us>, 
	Simon Horman <horms@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>, 
	Lorenzo Bianconi <lorenzo@kernel.org>, Coco Li <lixiaoyan@google.com>, Wei Wang <weiwan@google.com>, 
	Alexander Duyck <alexanderduyck@fb.com>, Hannes Frederic Sowa <hannes@stressinduktion.org>, 
	linux-kernel@vger.kernel.org, rcu@vger.kernel.org, bpf@vger.kernel.org, 
	kernel-team@cloudflare.com
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Hi Eric,

On Tue, Feb 27, 2024 at 10:44=E2=80=AFAM Eric Dumazet <edumazet@google.com>=
 wrote:
>
> Hmm....
> Why napi_busy_loop() does not have a similar problem ?
>
I just tried and can reproduce similar behavior on sk busy poll.
However, the interesting thing is, this can happen if I set a super
high polling interval but just send rare packets. In my case I had a 5
sec polling interval (unlikely to be realistic in prod but just for
demonstration), then used nc to send a few packets. Here is what
bpftrace react:

Normal:
time sudo bpftrace -e 'kfunc:napi_busy_loop{@=3Dcount();}
interval:s:1{exit();} kfunc:udp_recvmsg {printf("%ld\n",
args->sk->sk_ll_usec);}'
Attaching 3 probes...

@: 0

real    0m1.527s
user    0m0.073s
sys     0m0.128s


Extra wait when polling:
time sudo bpftrace -e 'kfunc:napi_busy_loop{@=3Dcount();}
interval:s:1{exit();} kfunc:udp_recvmsg {printf("%ld\n",
args->sk->sk_ll_usec);}'
Attaching 3 probes...
5000000


@: 16

real    0m11.167s
user    0m0.070s
sys     0m0.120s

So the symptoms are the same, bpftrace cannot exit despite having an
1sec timeout. But the execution pattern for these two are probably
different: NAPI threads would keep polling by itself, whereas sk poll
program might only poll when there is no immediate data. When there
are packets, it switches to process packets instead of polling any
more.


Yan