Hi Eric,
On Tue, Feb 27, 2024 at 10:44 AM Eric Dumazet <edumazet@xxxxxxxxxx> 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{@=count();}
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{@=count();}
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
