netdev - Re: [Xen-devel] [PATCH net] xen-netback: add the scenario which now beyond the range time_after

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Date:	Fri, 18 Oct 2013 00:21:22 +0800
From:	annie li <annie.li@...cle.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	Jason Luan <jianhai.luan@...cle.com>, david.vrabel@...rix.com,
	ian.campbell@...rix.com, wei.liu2@...rix.com,
	xen-devel@...ts.xenproject.org, netdev@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH net] xen-netback: add the scenario which now
 beyond the range time_after_eq().


On 2013-10-17 16:26, Jan Beulich wrote:
>>>> On 16.10.13 at 19:22, Jason Luan<jianhai.luan@...cle.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 am not so sure your mean about extending from 240+ to 480+. Do you 
mean "now" wrapped case happens and falls into the range of from expires 
to next_credit? If this happens, the timer would be set with value based 
on next_credit, which is actually implements the rate control.

Thanks
Annie
> I.e. why don't you simply
> change to 64-bit jiffy values, and use time_after_eq64()?
>
> Jan
>
>> Signed-off-by: Jason Luan<jianhai.luan@...cle.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@...ts.xen.org  
>> http://lists.xen.org/xen-devel  
>

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