Subject: Re: DomU's network interface will hung when Dom0 running 32bit

From: jianhai luan <[email-masked]>
Date: Tue, 15 Oct 2013 19:26:31 +0800

On 2013-10-15 18:06, Wei Liu wrote:
> On Tue, Oct 15, 2013 at 05:34:57PM +0800, jianhai luan wrote:
>> On 2013-10-15 16:43, Ian Campbell wrote:
>>> On Tue, 2013-10-15 at 10:44 +0800, jianhai luan wrote:
>>>> On 2013-10-14 19:19, Wei Liu wrote:
>>>>> On Sat, Oct 12, 2013 at 04:53:18PM +0800, jianhai luan wrote:
>>>>>> Hi Ian,
>>>>>>     I meet the DomU's network interface hung issue recently, and have
>>>>>> been working on the issue from that time. I find that DomU's network
>>>>>> interface, which send lesser package, will hung if Dom0 running
>>>>>> 32bit and DomU's up-time is very long.  I think that one jiffies
>>>>>> overflow bug exist in the function tx_credit_exceeded().
>>>>>>     I know the inline function time_after_eq(a,b) will process jiffies
>>>>>> overflow, but the function have one limit a should little that (b +
>>>>>> MAX_SIGNAL_LONG). If a large than the value, time_after_eq will
>>>>>> return false. The MAX_SINGNAL_LONG should be 0x7fffffff at 32-bit
>>>>>> machine.
>>>>>>     If DomU's network interface send lesser package (<0.5k/s if
>>>>>> jiffies=250 and credit_bytes=ULONG_MAX), jiffies will beyond out
>>>>>> (credit_timeout.expires + MAX_SIGNAL_LONG) and time_after_eq(now,
>>>>>> next_credit) will failure (should be true). So one timer which will
>>>>>> not be trigger in short time, and later process will be aborted when
>>>>>> timer_pending(&vif->credit_timeout) is true. The result will be
>>>>>> DomU's network interface will be hung in long time (> 40days).
>>>>>>     Please think about the below scenario:
>>>>>>     Condition:
>>>>>>       Dom0 running 32-bit and HZ = 1000
>>>>>>       vif->credit_timeout->expire = 0xffffffff, vif->remaining_credit
>>>>>> = 0xffffffff, vif->credit_usec=0 jiffies=0
>>>>>>       vif receive lesser package (DomU send lesser package). If the
>>>>>> value is litter than 2K/s, consume 4G(0xffffffff) will need 582.55
>>>>>> hours. jiffies will large than 0x7ffffff. we guess jiffies =
>>>>>> 0x800000ff, time_after_eq(0x800000ff, 0xffffffff) will failure, and
>>>>>> one time which expire is 0xfffffff will be pended into system. So
>>>>>> the interface will hung until jiffies recount 0xffffffff (that will
>>>>>> need very long time).
>>>>> If I'm not mistaken you meant time_after_eq(now, next_credit) in
>>>>> netback. How does next_credit become 0xffffffff?
>>>> I only assume the value is 0xfffffff, and the value of next_credit
>>>> isn't  point. If the delta between now and next_credit larger than
>>>> ULONG_MAX, time_after_eq will do wrong judge.
>>> So it sounds like we need a timer which is independent of the traffic
>>> being sent to keep credit_timeout.expires rolling over.
>>>
>>> Can you propose a patch?
>> Because credit_timeout.expire always after jiffies, i judge the
>> value over the range of time_after_eq() by time_before(now,
>> vif->credit_timeout.expires). please check the patch.
> I don't think this really fix the issue for you. You still have chance
> that now wraps around and falls between expires and next_credit. In that
> case it's stalled again.

if time_before(now, vif->credit_timeout.expires) is true, time wrap and 
do operation. Otherwise time_before(now, vif->credit_timeout.expires) 
isn't true, now - vif->credit_timeout.expires should be letter than 
ULONG_MAX/2. Because next_credit large than vif->credit_timeout.expires 
(next_crdit = vif->credit_timeout.expires + 
msecs_to_jiffies(vif->credit_usec/1000)), the delta between now and 
next_credit should be in range of time_after_eq().  So time_after_eq() 
do correctly judge.

Jason
> Wei.