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

From: Hannes Reinecke <hare@suse.de>
To: KY Srinivasan <kys@microsoft.com>,
	James Bottomley <James.Bottomley@HansenPartnership.com>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
	"ohering@suse.com" <ohering@suse.com>,
	"jbottomley@parallels.com" <jbottomley@parallels.com>,
	"hch@infradead.org" <hch@infradead.org>,
	"linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
	"apw@canonical.com" <apw@canonical.com>,
	"vkuznets@redhat.com" <vkuznets@redhat.com>,
	"jasowang@redhat.com" <jasowang@redhat.com>,
	"martin.petersen@oracle.com" <martin.petersen@oracle.com>
Subject: Re: [PATCH V3 4/4] scsi: storvsc: Tighten up the interrupt path
Date: Mon, 21 Dec 2015 08:42:08 +0100	[thread overview]
Message-ID: <5677AD50.4030406@suse.de> (raw)
In-Reply-To: <BY2PR0301MB1654C8B001A86200B918B9DFA0E20@BY2PR0301MB1654.namprd03.prod.outlook.com>

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@suse.com>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
hare@suse.de			               +49 911 74053 688
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