Re: [PATCH v1] clk: Convert managed get functions to devm_add_action API

[Robin Murphy]

On 02/12/2019 9:25 am, Marc Gonzalez wrote:
> On 02/12/2019 02:42, Dmitry Torokhov wrote:
>
> > On Thu, Nov 28, 2019 at 10:56:30AM -0800, Bjorn Andersson wrote:
> >
> > > On Tue 26 Nov 08:13 PST 2019, Marc Gonzalez wrote:
> > >
> > > > Date: Tue, 26 Nov 2019 13:56:53 +0100
> > > >
> > > > Using devm_add_action_or_reset() produces simpler code and smaller
> > > > object size:
> > > >
> > > > 1 file changed, 16 insertions(+), 46 deletions(-)
> > > >
> > > >      text	   data	    bss	    dec	    hex	filename
> > > > -   1797	     80	      0	   1877	    755	drivers/clk/clk-devres.o
> > > > +   1499	     56	      0	   1555	    613	drivers/clk/clk-devres.o
> > > >
> > > > Signed-off-by: Marc Gonzalez <marc.w.gonzalez@xxxxxxx>
> > >
> > > Looks neat
> > >
> > > Reviewed-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>
> >
> > This however increases the runtime costs as each custom action cost us
> > an extra pointer. Given that in a system we likely have many clocks
> > managed by devres, I am not sure that this code savings is actually
> > gives us overall win. It might still, I just want to understand how we
> > are allocating/packing devres structures.
>
> I'm not 100% sure what you are saying.

You reduce the text size by a constant amount, at the cost of allocating 
twice as much runtime data per clock (struct action_devres  vs. void*). 
Assuming 64-bit pointers, that means that in principle your ~320-byte 
saving would be cancelled out at ~40 managed clocks. However, that's 
also assuming that the minimum allocation granularity is no larger than 
a single pointer, which generally isn't true, so in reality it depends 
on whether the difference in data pushes the total struct devres 
allocation over the next ARCH_KMALLOC_MINALIGN boundary - if it doesn't, 
the difference comes entirely for free; if it does, the memory cost 
tradeoff gets even worse.

Robin.

> Are you arguing that the proposed patch increases the run-time cost of
> devm_clk_put() so much that the listed improvements (simpler source code,
> smaller object size) are not worth it?
>
> AFAIU, the release action is only called
> - explicitly, when devm_clk_put() is called
> - implicitly, when the device is removed
>
> How often are clocks removed?
>
> In hot code-path (called hundreds of times per second) it makes sense to
> write more complex code, to shave a few cycles every iteration. But in
> cold code-path, I think it's better to write short/simple code.
>
> Regards.
>
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