netdev - Re: [PATCH net] sctp: not allow to set rto

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Date:   Mon, 4 Jun 2018 20:15:17 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Michael Tuexen <michael.tuexen@...chi.franken.de>,
        Neil Horman <nhorman@...driver.com>,
        Netdev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
        David Miller <davem@...emloft.net>,
        David Ahern <dsa@...ulusnetworks.com>,
        Eric Dumazet <edumazet@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] sctp: not allow to set rto_min with a value below 200 msecs

On Mon, Jun 4, 2018 at 4:34 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Tue, May 29, 2018 at 7:45 PM, Xin Long <lucien.xin@...il.com> wrote:
>> On Wed, May 30, 2018 at 1:06 AM, Marcelo Ricardo Leitner
>> <marcelo.leitner@...il.com> wrote:
>>> On Tue, May 29, 2018 at 12:03:46PM -0400, Neal Cardwell wrote:
>>>> On Tue, May 29, 2018 at 11:45 AM Marcelo Ricardo Leitner <
>>>> marcelo.leitner@...il.com> wrote:
>>>> > - patch2 - fix rtx attack vector
>>>> >    - Add the floor value to rto_min to HZ/20 (which fits the values
>>>> >      that Michael shared on the other email)
>>>>
>>>> I would encourage allowing minimum RTO values down to 5ms, if the ACK
>>>> policy in the receiver makes this feasible. Our experience is that in
>>>> datacenter environments it can be advantageous to allow timer-based loss
>>>> recoveries using timeout values as low as 5ms, e.g.:
>>>
>>> Thanks Neal. On Xin's tests, the hearbeat timer becomes an issue at
>>> ~25ms already. Xin, can you share more details on the hw, which CPU
>>> was used?
>
> Hi,
>
> Did we reach any decision on this? This continues to produce bug
> reports on syzbot.
I will post a patch later today for the suggestion:
- patch1 - fix issue at hand
  - Use the max_t above
to fix this.


As for patch2 and patch3:
- patch2 - fix rtx attack vector
  - Add the floor value to rto_min to HZ/20 (which fits the values
    that Michael shared on the other email)
- patch3 - speed up initial HB again
  - change sctp_cmd_hb_timers_start() so hb timers are kickstarted
    when the association is established. AFAICT RFC doesn't specify
    when these initial ones should be sent, and I see no issues with
    speeding them up.

They are more like improvements, we will do it in the future after
getting more information.


>
> I am not sure whom you are asking, because Xin is you unless I am
> missing something :)
> But if you mean syzbot hardware, then it's GCE VMs with modern Intel
> CPUs but an important aspect is a heavy-debug config (which you can
> take from here https://syzkaller.appspot.com/bug?extid=3dcd59a1f907245f891f)
> and systematic bug reporting. So if it's any flaky in your testing, it
> will produce dozens of bug emails on syzbot.
>
>
>> It was on a KVM guest,  "-smp 2,cores=1,threads=1,sockets=2"
>> # lscpu
>> Architecture:          x86_64
>> CPU op-mode(s):        32-bit, 64-bit
>> Byte Order:            Little Endian
>> CPU(s):                2
>> On-line CPU(s) list:   0,1
>> Thread(s) per core:    1
>> Core(s) per socket:    1
>> Socket(s):             2
>> NUMA node(s):          1
>> Vendor ID:             GenuineIntel
>> CPU family:            6
>> Model:                 13
>> Model name:            QEMU Virtual CPU version 1.5.3
>> Stepping:              3
>> CPU MHz:               2397.222
>> BogoMIPS:              4794.44
>> Hypervisor vendor:     KVM
>> Virtualization type:   full
>> L1d cache:             32K
>> L1i cache:             32K
>> L2 cache:              4096K
>> NUMA node0 CPU(s):     0,1
>> Flags:                 fpu de pse tsc msr pae mce cx8 apic sep mtrr
>> pge mca cmov pse36 clflush mmx fxsr sse sse2 syscall nx lm rep_good
>> nopl cpuid pni cx16 hypervisor lahf_lm abm pti
>>
>> If we're counting on max_t to fix this CPU stuck. It should not that
>> matter if min rto < the value causing that stuck.
>>
>>>
>>> Anyway, what about we add a floor to rto_max too, so that RTO can
>>> actually grow into something bigger that don't hog the CPU? Like:
>>> rto_min floor = 5ms
>>> rto_max floor = 50ms
>>>
>>>   Marcelo