Re: Re: Re: Latency spikes occurs from frequent socket connections

[sjpark]

On Thu, 30 Jan 2020 09:02:08 -0800 Eric Dumazet <edumazet@xxxxxxxxxx> wrote:

> On Thu, Jan 30, 2020 at 4:41 AM <sjpark@xxxxxxxxxx> wrote:
> >
> > On Wed, 29 Jan 2020 09:52:43 -0800 Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> > > On Wed, Jan 29, 2020 at 9:14 AM <sjpark@xxxxxxxxxx> wrote:
> > > >
> > > > Hello,
> > > >
> > > >
> > > > We found races in the kernel code that incur latency spikes.  We thus would
> > > > like to share our investigations and hear your opinions.
> > > >
[...]
> > >
> > > I would rather try to fix the issue more generically, without adding
> > > extra lookups as you did, since they might appear
> > > to reduce the race, but not completely fix it.
> > >
> > > For example, the fact that the client side ignores the RST and
> > > retransmits a SYN after one second might be something that should be
> > > fixed.
> >
> > I also agree with this direction.  It seems detecting this situation and
> > adjusting the return value of tcp_timeout_init() to a value much lower than the
> > one second would be a straightforward solution.  For a test, I modified the
> > function to return 1 (4ms for CONFIG_HZ=250) and confirmed the reproducer be
> > silent.  My following question is, how we can detect this situation in kernel?
> > However, I'm unsure how we can distinguish this specific case from other cases,
> > as everything is working as normal according to the TCP protocol.
> >
> > Also, it seems the value is made to be adjustable from the user space using the
> > bpf callback, BPF_SOCK_OPS_TIMEOUT_INIT:
> >
> >     BPF_SOCK_OPS_TIMEOUT_INIT,  /* Should return SYN-RTO value to use or
> >                                  * -1 if default value should be used
> >                                  */
> >
> > Thus, it sounds like you are suggesting to do the detection and adjustment from
> > user space.  Am I understanding your point?  If not, please let me know.
> >
> 
> No, I was suggesting to implement a mitigation in the kernel :
> 
> When in SYN_SENT state, receiving an suspicious ACK should not
> simply trigger a RST.
> 
> There are multiple ways maybe to address the issue.
> 
> 1) Abort the SYN_SENT state and let user space receive an error to its
> connect() immediately.
> 
> 2) Instead of a RST, allow the first SYN retransmit to happen immediately
> (This is kind of a challenge SYN. Kernel already implements challenge acks)
> 
> 3) After RST is sent (to hopefully clear the state of the remote),
> schedule a SYN rtx in a few ms,
> instead of ~ one second.

Thank you for this kind comment, Eric!  I would prefer the second and third
idea rather than first one.  Anyway, I will send a patch soon.  Will add a
kselftest for this case, too.


Thanks,
SeongJae Park

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