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

From: Robin Murphy <robin.murphy@arm.com>
To: Marc Gonzalez <marc.w.gonzalez@free.fr>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>,
	Stephen Boyd <sboyd@kernel.org>,
	Michael Turquette <mturquette@baylibre.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
	Russell King <rmk+kernel@armlinux.org.uk>,
	Guenter Roeck <linux@roeck-us.net>,
	linux-clk <linux-clk@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v1] clk: Convert managed get functions to devm_add_action API
Date: Mon, 2 Dec 2019 13:51:08 +0000	[thread overview]
Message-ID: <c0ccca86-b7b1-b587-60c1-4794376fa789@arm.com> (raw)
In-Reply-To: <f177ef95-ef7e-cab0-1322-6de28f18ecdb@free.fr>

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.
> 
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