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

[Hannes Reinecke]

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@xxxxxxxx>

Cheers,

Hannes
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Dr. Hannes Reinecke		               zSeries & Storage
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