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SubjectRe: [PATCH v1] clk: Convert managed get functions to devm_add_action API
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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@free.fr>
>>>
>>> Looks neat
>>>
>>> Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
>>
>> 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.
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>

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