Re: [PATCH net] sctp: not allow to set rto_min with a value below 200 msecs

From: Dmitry Vyukov <dvyukov@google.com>
To: Xin Long <lucien.xin@gmail.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>,
	Neal Cardwell <ncardwell@google.com>,
	Michael Tuexen <michael.tuexen@lurchi.franken.de>,
	Neil Horman <nhorman@tuxdriver.com>,
	Netdev <netdev@vger.kernel.org>,
	linux-sctp@vger.kernel.org, David Miller <davem@davemloft.net>,
	David Ahern <dsa@cumulusnetworks.com>,
	Eric Dumazet <edumazet@google.com>,
	syzkaller <syzkaller@googlegroups.com>
Subject: Re: [PATCH net] sctp: not allow to set rto_min with a value below 200 msecs
Date: Mon, 4 Jun 2018 10:34:07 +0200	[thread overview]
Message-ID: <CACT4Y+bVvSsQm0hywC-_UqnJKhBeQryZZBjYaiWszvQGURS=vA@mail.gmail.com> (raw)
In-Reply-To: <CADvbK_fbKbH2wm6Xurr+ELVag-LvyQdL+peJd=wp7OL7_zMZTQ@mail.gmail.com>

On Tue, May 29, 2018 at 7:45 PM, Xin Long <lucien.xin@gmail.com> wrote:
> On Wed, May 30, 2018 at 1:06 AM, Marcelo Ricardo Leitner
> <marcelo.leitner@gmail.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@gmail.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 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