netdev - Re: [PATCH 1/8] ixgbe: Out of line ixgbe_read/write

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Date:	Tue, 20 May 2014 17:06:17 +0000
From:	"Rustad, Mark D" <mark.d.rustad@...el.com>
To:	Andi Kleen <ak@...ux.intel.com>
CC:	Andi Kleen <andi@...stfloor.org>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"<linux-kernel@...r.kernel.org>" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/8] ixgbe: Out of line ixgbe_read/write_reg

On May 19, 2014, at 4:25 PM, Andi Kleen <ak@...ux.intel.com> wrote:

> On Mon, May 19, 2014 at 10:00:52PM +0000, Rustad, Mark D wrote:
>> On May 16, 2014, at 2:43 PM, Andi Kleen <andi@...stfloor.org> wrote:
>> 
>>> From: Andi Kleen <ak@...ux.intel.com>
>>> 
>>> ixgbe_read_reg and ixgbe_write_reg are frequently called and are very big
>>> because they have complex error handling code.
>> 
>> Actually, this patch doesn't do anything to ixgbe_write_reg, which would almost certainly be very bad for performance, but instead changes ixgbe_write_reg64.
> 
> I doubt a few cycles around the write make a lot of difference for MMIO. MMIO is dominated
> by other things.
> 
>> The latter is not in a performance-sensitive path, but is only called from one site, so there is little reason to take it out-of-line.
> 
> True I moved the wrong one.
> 
> ixgbe_write_reg                                    3305  (0.00%)  8     409  
> 
> 
>> I already have a patch in queue to make ixgbe_read_reg out-of-line, because it does have a very costly memory footprint inline, as you have found.
> 
> Please move write_reg too.

I will take a look at moving most of them out-of-line. There are just a few in very hot paths that should remain inline.

-- 
Mark Rustad, Networking Division, Intel Corporation

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