[PATCH V3 4/4] scsi: storvsc: Tighten up the interrupt path
Hannes Reinecke
hare at suse.de
Mon Dec 21 07:42:08 UTC 2015
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 at suse.com>
Cheers,
Hannes
--
Dr. Hannes Reinecke zSeries & Storage
hare at suse.de +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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