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

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Message-ID: <525D6C16.6010705@oracle.com>
Date:	Wed, 16 Oct 2013 00:23:50 +0800
From:	jianhai luan <jianhai.luan@...cle.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	Ian Campbell <Ian.Campbell@...rix.com>,
	xen-devel@...ts.xenproject.org, netdev@...r.kernel.org,
	ANNIE LI <annie.li@...cle.com>
Subject: Re: DomU's network interface will hung when Dom0 running 32bit


On 2013-10-16 0:03, Wei Liu wrote:
> On Tue, Oct 15, 2013 at 11:19:42PM +0800, jianhai luan wrote:
> [...]
>>>>>> * time_after_eq(now, next_credit) -> false
>>>>>> * time_before(now, expires) -> false
>>>>> If now is placed in above environment, the result will be correct
>>>>> (Sending package will be not allowed until next_credit).
>>>> No, it is not necessarily correct. Keep in mind that "now" wraps around,
>>>> which is the issue you try to fix. You still have a window to stall your
>>>> frontend.
>>> Remember that time_after_eq is supposed to work even with wraparound
>>> occurring, so long as the two times are less than MAX_LONG/2 apart.
>> Sorry for my misunderstand explanation. I mean that
>>    * time_after_eq()/time_before_eq() fix the jiffies wraparound, so
>> please think about  jiffies in line increasing.
>>    * time_after_eq()/time_before_eq() have the range (0, MAX_LONG/2),
>> the judge will be wrong if out of the range.
>>
>> So please think about three kind environment
>>    -  expires        now        next_credit
>>       --------time increases this direction ---------->
>>
>>    -  expires        [next_credit        now next_credit+MAX_LONG/2
>>       --------time increase this direction ----------->
>>
>>    - expires        next_credit        next_credit+MAX_LONG/2 now
>>       --------time increadse this direction ---------->
>>
>> The first environment should be netfront consume all credit_byte
>> before next_credit, So we should pending one timer to calculator the
>> new credit_byte, and don't transmit until next_credit.
>>
>> the second environment should be calculator the credit_byte because
>> netfront don't consume all credit_byte before next_credit, and
>> time_after_eq() do correct judge.
>>
>> the third environment should be calculator in time because netfront
>> don't consume all credit_byte until next_credit.But time_after_eq do
>> error judge (time_after_eq(now, next_credit) is false), so the
>> remaining_byte isn't be increased.
>>
>> and I work on the third environment.  You know now >
>> next_credit+MAX_LONG/2, time_before(now, expire) should be
>> true(time_before(now, expire) is false in first environment)
> Thanks for staighten this out for me. I'm just too dumb for this, please
> be patient with me. :-)
>
> Could you prove that time_before(now, expire) is always true in third
> case? That's where my main cencern lies. Is it because msecs_to_jiffies
> always returns MAX_JIFFY_OFFSET (which is ((LONG_MAX >> 1)-1) ) at most?

I have wrong judge in third environment. If now large than expires + 
MAX_UNLONG, time_before(now, expires) will be false.
   expires    next_credit    next_credit+MAX_UNLONG/2    expires + 
MAX_UNLONG    now    next_credit+MAX_UNLONG
   --------------------------------------------------------- time 
increadse this direction  ---------------------------------->

   In the above environment, time_before(now, expires) will return 
false. But the jiffies elapsed more time and next_credit will be 
reachable in soon(time_after_eq(now, next_credit) will be true).
>
> Wei.

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