linux-kernel - Re: [PATCH net-next 1/2] net: remove NET_LL_RX

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Date:	Thu, 13 Jun 2013 10:00:26 +0200
From:	Daniel Borkmann <dborkman@...hat.com>
To:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
CC:	Stephen Hemminger <stephen@...workplumber.org>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	jesse.brandeburg@...el.com, donald.c.skidmore@...el.com,
	e1000-devel@...ts.sourceforge.net, willemb@...gle.com,
	erdnetdev@...il.com, bhutchings@...arflare.com,
	andi@...stfloor.org, hpa@...or.com, eilong@...adcom.com,
	or.gerlitz@...il.com, amirv@...lanox.com, eliezer@...ir.org.il
Subject: Re: [PATCH net-next 1/2] net: remove NET_LL_RX_POLL config menue

On 06/13/2013 04:13 AM, Eliezer Tamir wrote:
> On 13/06/2013 05:01, Stephen Hemminger wrote:
>> On Wed, 12 Jun 2013 15:12:05 -0700 (PDT)
>> David Miller <davem@...emloft.net> wrote:
>>
>>> From: Eliezer Tamir <eliezer.tamir@...ux.intel.com>
>>> Date: Tue, 11 Jun 2013 17:24:28 +0300
>>>
>>>>       depends on X86_TSC
>>>
>>> Wait a second, I didn't notice this before.  There needs to be a better
>>> way to test for the accuracy you need, or if the issue is lack of a proper
>>> API for cycle counter reading, fix that rather than add ugly arch
>>> specific dependencies to generic networking code.
>>
>> This should be sched_clock(), rather than direct TSC access.
>> Also any code using TSC or sched_clock has to be carefully audited to deal with
>> clocks running at different rates on different CPU's. Basically value is only
>> meaning full on same CPU.
>
> OK,
>
> If we covert to sched_clock(), would adding a define such as HAVE_HIGH_PRECISION_CLOCK to architectures that have both a high precision clock and a 64 bit cycles_t be a good solution?
>
> (if not any other suggestion?)

Hm, probably cpu_clock() and similar might be better, since they use
sched_clock() in the background when !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
(meaning when sched_clock() provides synchronized highres time source from
the architecture), and, quoting ....

  Otherwise it tries to create a semi stable clock from a mixture of other
  clocks, including:

   - GTOD (clock monotomic)
   - sched_clock()
   - explicit idle events

But yeah, it needs to be evaluated regarding the drift between CPUs in
general.

Then, eventually, you could get rid of the entire NET_LL_RX_POLL config
option plus related ifdefs in the code and have it built-in in general?
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