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

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Message-Id: <5268025002000078000A56B5@nat28.tlf.novell.com>
Date:	Wed, 23 Oct 2013 17:07:28 +0100
From:	"Jan Beulich" <jbeulich@...e.com>
To:	<David.Laight@...LAB.COM>, <wei.liu2@...rix.com>,
	<jianhai.luan@...cle.com>
Cc:	<david.vrabel@...rix.com>, <ian.campbell@...rix.com>,
	<xen-devel@...ts.xenproject.org>, <annie.li@...cle.com>,
	<netdev@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH net] xen-netback: add the scenario
 which now beyond the range time_after_eq().

>>> jianhai luan <jianhai.luan@...cle.com> 10/23/13 10:02 AM >>>
>On 2013-10-18 19:24, Wei Liu wrote:
>> On Fri, Oct 18, 2013 at 09:40:33AM +0100, David Laight wrote:
>>>>> My understanding is this patch does not simply double the span, it is
>>>>> just stricter than the original one. Please check my previous comments,
>>>>> I paste it here.
>>>> No, the code (on a 32-bit arch) just _can't_ handle jiffies differences
>>>> beyond 2^32, no matter how cleverly you use the respective macros.
>>>> All arithmetic there is done modulo 2^32.
>>> I haven't followed this discussion very closely but it might be possible
>>> to arrange that the 'incorrect lack of credit' only occurs for a few
>>> seconds every time 'jiffies' wraps - instead of half of the time.
>>> Then you'd have to be extremely unlucky to hit the timing window.
>>>
>> As I understand it, this is the idea of this patch -- to narrow down the
>> timing window.
>Jan,  do you agree the idea or have better suggestion to me.

As said before - I disagree (reducing a timing window is never a solution,
only eliminating it is), and I pointed at the alternative (using 64-bit
calculations) before.

Jan

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