linux-kernel - Re: [PATCH V3 4/4] scsi: storvsc: Tighten up the interrupt path

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Date:	Mon, 21 Dec 2015 08:42:08 +0100
From:	Hannes Reinecke <hare@...e.de>
To:	KY Srinivasan <kys@...rosoft.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"ohering@...e.com" <ohering@...e.com>,
	"jbottomley@...allels.com" <jbottomley@...allels.com>,
	"hch@...radead.org" <hch@...radead.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"apw@...onical.com" <apw@...onical.com>,
	"vkuznets@...hat.com" <vkuznets@...hat.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	"martin.petersen@...cle.com" <martin.petersen@...cle.com>
Subject: Re: [PATCH V3 4/4] scsi: storvsc: Tighten up the interrupt path

On 12/19/2015 03:28 AM, KY Srinivasan wrote:
>
[ .. ]
>>
>> Could you?  You're making what you describe as an optimisation but
>> there are two reasons why this might not be so.  The first is that the
>> compiler is entitled to inline static functions.  If it did, likely it
>> picked up the optmisation anyway as Hannes suggested.  However, the
>> other reason this might not be an optimisation (assuming the compiler
>> doesn't inline the function) is you're passing an argument which can be
>> offset computed.  On all architectures, you have a fixed number of
>> registers for passing function arguments, then we have to use the
>> stack.  Using the stack comes in far more expensive than computing an
>> offset to an existing pointer.  Even if you're still in registers, the
>> offset now has to be computed and stored and the compiler loses track
>> of the relation.
>>
>> The bottom line is that adding an extra argument for a value which can
>> be offset computed is rarely a win.
>
> James,
> When I did this, I was mostly concerned about the cost of reestablishing state that was
> already known. So, even with the function being in-lined, I felt the cost of reestablishing
> state that was already known is unnecessary. In this particular case, I did not change the
> number of arguments that were being passed; I just changed the type of one of them -
> instead of passing struct hv_device *, I am now passing struct storvsc_device *. In the
> current code, we are using struct hv_device * to establish a pointer to struct storvsc_device *
> via the function get_in_stor_device(). This pattern currently exists in the call chain from the
> interrupt handler - storvsc_on_channel_callback().
>
> While the compiler is smart enough to inline both get_in_stor_device() as well as many of the static
> functions in the call chain from storvsc_on_channel_callback(), looking at the assembled code,
> the compiler is repeatedly inlining the call to get_in_stor_device() and this clearly is less than optimal.
>
Which means you actually checked the compiler output, and it made a 
difference.

That's all I wanted to know, as it's not immediately clear from the 
patch.

So:

Reviewed-by: Hannes Reinecke <hare@...e.com>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
hare@...e.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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HRB 21284 (AG Nürnberg)
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