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Date:   Sat, 19 Feb 2022 17:22:13 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Olivier Langlois <olivier@...llion01.com>
Cc:     Pavel Begunkov <asml.silence@...il.com>,
        Hao Xu <haoxu@...ux.alibaba.com>,
        io-uring <io-uring@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] io_uring: Add support for napi_busy_poll

On 2/19/22 2:42 PM, Olivier Langlois wrote:
> One side effect that I have discovered from testing the napi_busy_poll
> patch, despite improving the network timing of the threads performing
> the busy poll, it is the networking performance degradation that it has
> on the rest of the system.
> 
> I dedicate isolated CPUS to specific threads of my program. My kernel
> is compiled with CONFIG_NO_HZ_FULL. One thing that I have never really
> understood is why there were still kernel threads assigned to the
> isolated CPUs.
> 
> $ CORENUM=2; ps -L -e -o pid,psr,cpu,cmd | grep -E 
> "^[[:space:]]+[[:digit:]]+[[:space:]]+${CORENUM}"
>      24   2   - [cpuhp/2]
>      25   2   - [idle_inject/2]
>      26   2   - [migration/2]
>      27   2   - [ksoftirqd/2]
>      28   2   - [kworker/2:0-events]
>      29   2   - [kworker/2:0H]
>      83   2   - [kworker/2:1-mm_percpu_wq]
> 
> It is very hard to keep the CPU 100% tickless if there are still tasks
> assigned to isolated CPUs by the kernel.
> 
> This question isn't really answered anywhere AFAIK:
> https://www.kernel.org/doc/html/latest/timers/no_hz.html
> https://jeremyeder.com/2013/11/15/nohz_fullgodmode/
> 
> Those threads running on their dedicated CPUS are the ones doing the
> NAPI busy polling. Because of that, those CPUs usage ramp up to 100%
> and running ping on the side is now having horrible numbers:
> 
> [2022-02-19 07:27:54] INFO SOCKPP/ping ping results for 10 loops:
> 0. 104.16.211.191 rtt min/avg/max/mdev = 9.926/34.987/80.048/17.016 ms
> 1. 104.16.212.191 rtt min/avg/max/mdev = 9.861/34.934/79.986/17.019 ms
> 2. 104.16.213.191 rtt min/avg/max/mdev = 9.876/34.949/79.965/16.997 ms
> 3. 104.16.214.191 rtt min/avg/max/mdev = 9.852/34.927/79.977/17.019 ms
> 4. 104.16.215.191 rtt min/avg/max/mdev = 9.869/34.943/79.958/16.997 ms
> 
> Doing this:
> echo 990000 > /proc/sys/kernel/sched_rt_runtime_us
> 
> as instructed here:
> https://www.kernel.org/doc/html/latest/scheduler/sched-rt-group.html
> 
> fix the problem:
> 
> $ ping 104.16.211.191
> PING 104.16.211.191 (104.16.211.191) 56(84) bytes of data.
> 64 bytes from 104.16.211.191: icmp_seq=1 ttl=62 time=1.05 ms
> 64 bytes from 104.16.211.191: icmp_seq=2 ttl=62 time=0.812 ms
> 64 bytes from 104.16.211.191: icmp_seq=3 ttl=62 time=0.864 ms
> 64 bytes from 104.16.211.191: icmp_seq=4 ttl=62 time=0.846 ms
> 64 bytes from 104.16.211.191: icmp_seq=5 ttl=62 time=1.23 ms
> 64 bytes from 104.16.211.191: icmp_seq=6 ttl=62 time=0.957 ms
> 64 bytes from 104.16.211.191: icmp_seq=7 ttl=62 time=1.10 ms
> ^C
> --- 104.16.211.191 ping statistics ---
> 7 packets transmitted, 7 received, 0% packet loss, time 6230ms
> rtt min/avg/max/mdev = 0.812/0.979/1.231/0.142 ms
> 
> If I was to guess, I would say that it is ksoftirqd on those CPUs that
> is starving and is not servicing the network packets but I wish that I
> had a better understanding of what is really happening and know if it
> would be possible to keep 100% those processors dedicated to my tasks
> and have the network softirqs handled somewhere else to not have to
> tweak /proc/sys/kernel/sched_rt_runtime_us to fix the issue...

Outside of this, I was hoping to see some performance numbers in the
main patch. Sounds like you have them, can you share?

-- 
Jens Axboe

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