Re: [Xen-devel] [PATCH net] xen-netback: add the scenario which now beyond the range time_after_eq().

[jianhai luan <jianhai.luan@oracle.com>]

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On 2013-10-17 17:02, jianhai luan wrote:
>
> On 2013-10-17 16:26, Jan Beulich wrote:
>>>>> On 16.10.13 at 19:22, Jason Luan <jianhai.luan@oracle.com> wrote:
>>> time_after_eq() only works if the delta is < MAX_ULONG/2.
>>>
>>> If netfront sends at a very low rate, the time between subsequent calls
>>> to tx_credit_exceeded() may exceed MAX_ULONG/2 and the test for
>>> timer_after_eq() will be incorrect.  Credit will not be replenished and
>>> the guest may become unable to send (e.g., if prior to the long gap, 
>>> all
>>> credit was exhausted).
>>>
>>> We should add the scenario which now beyond 
>>> next_credit+MAX_UNLONG/2. Because
>>> the fact now must be not before than expire, time_before(now, 
>>> expire) == true
>>> will verify the scenario.
>>>      time_after_eq(now, next_credit) || time_before (now, expire)
>>>      ==
>>>      !time_in_range_open(now, expire, next_credit)
>> So first of all this must be with a 32-bit netback. And the not
>> coverable gap between activity is well over 240 days long. _If_
>> this really needs dealing with, then why is extending this from
>> 240+ to 480+ days sufficient? I.e. why don't you simply
>> change to 64-bit jiffy values, and use time_after_eq64()?
>
> Yes, the issue only can be  reproduced in 32-bit Dom0 (Beyond 
> MAX_ULONG/2 in 64-bit will need long long time)
>
> I think the gap should be think all environment even now extending 
> 480+. if now fall in the gap,  one timer will be pending and replenish 
> will be in time.  Please run the attachment test program.
>

Sorry for miss the attachment in previous letter. Please check the 
attachment.
> If use time_after_eq64(), expire ,next_credit and other member will 
> must be u64.
>>
>> Jan
>>
>>> Signed-off-by: Jason Luan <jianhai.luan@oracle.com>
>>> ---
>>>   drivers/net/xen-netback/netback.c |    7 +++++--
>>>   1 files changed, 5 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/net/xen-netback/netback.c
>>> b/drivers/net/xen-netback/netback.c
>>> index f3e591c..31eedaf 100644
>>> --- a/drivers/net/xen-netback/netback.c
>>> +++ b/drivers/net/xen-netback/netback.c
>>> @@ -1194,8 +1194,11 @@ static bool tx_credit_exceeded(struct xenvif 
>>> *vif,
>>> unsigned size)
>>>       if (timer_pending(&vif->credit_timeout))
>>>           return true;
>>>   -    /* Passed the point where we can replenish credit? */
>>> -    if (time_after_eq(now, next_credit)) {
>>> +    /* Credit should be replenished when now does not fall into the
>>> +     * range from expires to next_credit, and time_in_range_open()
>>> +     * is used to verify whether this case happens.
>>> +     */
>>> +    if (!time_in_range_open(now, vif->credit_timeout.expires, 
>>> next_credit)) {
>>>           vif->credit_timeout.expires = now;
>>>           tx_add_credit(vif);
>>>       }
>>> -- 
>>> 1.7.6.5
>>>
>>>
>>> _______________________________________________
>>> Xen-devel mailing list
>>> Xen-devel@lists.xen.org
>>> http://lists.xen.org/xen-devel
>>
>>
>


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#include <stdio.h>

#define typecheck(type,x) \
({	type __dummy; \
	typeof(x) __dummy2; \
	(void)(&__dummy == &__dummy2); \
	1; \
})

#define time_after(a, b)		\
	(typecheck(unsigned char, a) && \
	 typecheck(unsigned char, b) && \
	 ((char)((b) - (a)) < 0))
#define time_before(a,b)	time_after(b,a)

#define time_after_eq(a,b)		\
	(typecheck(unsigned char, a) && \
	 typecheck(unsigned char, b) && \
	 ((char)((a) -(b)) >= 0))
#define time_before_eq(a, b) time_after_eq(b,a)

void do_nothing()
{
	return;
}

int main()
{
	unsigned char expire, now, next;
	unsigned char delta = 10;
	int i, j;

	for(i = 0; i < 256; i++) {
		expire = i;
		next = expire + delta;

		printf("\n\n\n[%u ... %u]\n", expire, next);
		now = expire;
		for(j=0; j < 1024; j++, now++) {	
			if(j%256 == 0) printf("\n");

			if (time_after_eq(now, next) ||
				time_before(now, expire)) {
				do_nothing();
			}
			else {
				printf("    now=%d\n", (char)now);
			}
		}
	}
	
	return 0;
}