linux-kernel - Re: [PATCH net] udp: fix memory schedule error

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Date:   Thu, 23 Feb 2023 09:39:17 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Jason Xing <kerneljasonxing@...il.com>
Cc:     willemdebruijn.kernel@...il.com, davem@...emloft.net,
        dsahern@...nel.org, edumazet@...gle.com, kuba@...nel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net] udp: fix memory schedule error

On Wed, 2023-02-22 at 11:47 +0800, Jason Xing wrote:
> On Tue, Feb 21, 2023 at 11:46 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> > 
> > On Tue, Feb 21, 2023 at 10:46 PM Paolo Abeni <pabeni@...hat.com> wrote:
> > > 
> > > On Tue, 2023-02-21 at 21:39 +0800, Jason Xing wrote:
> > > > On Tue, Feb 21, 2023 at 8:27 PM Paolo Abeni <pabeni@...hat.com> wrote:
> > > > > 
> > > > > On Tue, 2023-02-21 at 19:03 +0800, Jason Xing wrote:
> > > > > > From: Jason Xing <kernelxing@...cent.com>
> > > > > > 
> > > > > > Quoting from the commit 7c80b038d23e ("net: fix sk_wmem_schedule()
> > > > > > and sk_rmem_schedule() errors"):
> > > > > > 
> > > > > > "If sk->sk_forward_alloc is 150000, and we need to schedule 150001 bytes,
> > > > > > we want to allocate 1 byte more (rounded up to one page),
> > > > > > instead of 150001"
> > > > > 
> > > > > I'm wondering if this would cause measurable (even small) performance
> > > > > regression? Specifically under high packet rate, with BH and user-space
> > > > > processing happening on different CPUs.
> > > > > 
> > > > > Could you please provide the relevant performance figures?
> > > > 
> > > > Sure, I've done some basic tests on my machine as below.
> > > > 
> > > > Environment: 16 cpus, 60G memory
> > > > Server: run "iperf3 -s -p [port]" command and start 500 processes.
> > > > Client: run "iperf3 -u -c 127.0.0.1 -p [port]" command and start 500 processes.
> > > 
> > > Just for the records, with the above command each process will send
> > > pkts at 1mbs - not very relevant performance wise.
> > > 
> > > Instead you could do:
> > > 
> > 
> > > taskset 0x2 iperf -s &
> > > iperf -u -c 127.0.0.1 -b 0 -l 64
> > > 
> > 
> > Thanks for your guidance.
> > 
> > Here're some numbers according to what you suggested, which I tested
> > several times.
> > ----------|IFACE   rxpck/s   txpck/s    rxkB/s    txkB/s
> > Before: lo 411073.41 411073.41  36932.38  36932.38
> > After:   lo 410308.73 410308.73  36863.81  36863.81
> > 
> > Above is one of many results which does not mean that the original
> > code absolutely outperforms.
> > The output is not that constant and stable, I think.
> 
> Today, I ran the same test on other servers, it looks the same as
> above. Those results fluctuate within ~2%.
> 
> Oh, one more thing I forgot to say is the output of iperf itself which
> doesn't show any difference.
> Before: Bitrate is 211 - 212 Mbits/sec
> After: Bitrate is 211 - 212 Mbits/sec
> So this result is relatively constant especially if we keep running
> the test over 2 minutes.

Thanks for the testing. My personal take on this one is that is more a
refactor than a bug fix - as the amount forward allocated memory should
always be negligible for UDP. 

Still it could make sense keep the accounting schema consistent across
different protocols. I suggest to repost for net-next, when it will re-
open, additionally introducing __sk_mem_schedule() usage to avoid code
duplication.

Thanks,

Paolo