Messages in this thread | | | Subject | Re: [PATCH v1] clk: Convert managed get functions to devm_add_action API | From | Marc Gonzalez <> | Date | Thu, 12 Dec 2019 17:59:04 +0100 |
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On 12/12/2019 15:47, Robin Murphy wrote:
> On 12/12/2019 1:53 pm, Marc Gonzalez wrote: > >> On 11/12/2019 23:28, Dmitry Torokhov wrote: >> >>> On Wed, Dec 11, 2019 at 05:17:28PM +0100, Marc Gonzalez wrote: >>> >>>> What is the rationale for the devm_add_action API? >>> >>> For one-off and maybe complex unwind actions in drivers that wish to use >>> devm API (as mixing devm and manual release is verboten). Also is often >>> used when some core subsystem does not provide enough devm APIs. >> >> Thanks for the insight, Dmitry. Thanks to Robin too. >> >> This is what I understand so far: >> >> devm_add_action() is nice because it hides/factorizes the complexity >> of the devres API, but it incurs a small storage overhead of one >> pointer per call, which makes it unfit for frequently used actions, >> such as clk_get. >> >> Is that correct? >> >> My question is: why not design the API without the small overhead? > > Probably because on most architectures, ARCH_KMALLOC_MINALIGN is at > least as big as two pointers anyway, so this "overhead" should mostly be > free in practice. Plus the devres API is almost entirely about being > able to write simple robust code, rather than absolute efficiency - I > mean, struct devres itself is already 5 pointers large at the absolute > minimum ;)
(3 pointers: 1 list_head + 1 function pointer)
I'm confused. The first patch was criticized for potentially adding an extra pointer for every devm_clk_get (e.g. 800 bytes on a 64-bit platform with 100 clocks).
Let's see. On arm64, ARCH_KMALLOC_MINALIGN is 128.
So basically, a struct devres looks like this on arm64:
list_head.next list_head.prev dr_release_t . . . 104 bytes of padding . . . data (flexible array) . . . padding up to 256 bytes
Basically, on arm64, every struct devres occupies 256 bytes, most of it (typically 104 + 112 = 216) wasted as padding.
Hmmm, given how many devm stuff goes on in a modern platform, there might be large savings to be had...
Assuming 10,000 calls to devres_alloc_node(), we would be wasting ~2 MB of RAM. Not sure it's worth trying to save that?
$ git grep '#define ARCH_DMA_MINALIGN' arch/arc/include/asm/cache.h:#define ARCH_DMA_MINALIGN SMP_CACHE_BYTES arch/arm/include/asm/cache.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES arch/arm64/include/asm/cache.h:#define ARCH_DMA_MINALIGN (128) arch/c6x/include/asm/cache.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES arch/csky/include/asm/cache.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES arch/hexagon/include/asm/cache.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES arch/m68k/include/asm/cache.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES arch/microblaze/include/asm/page.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES arch/mips/include/asm/mach-generic/kmalloc.h:#define ARCH_DMA_MINALIGN 128 arch/mips/include/asm/mach-ip32/kmalloc.h:#define ARCH_DMA_MINALIGN 32 arch/mips/include/asm/mach-ip32/kmalloc.h:#define ARCH_DMA_MINALIGN 128 arch/mips/include/asm/mach-tx49xx/kmalloc.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES arch/nds32/include/asm/cache.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES arch/nios2/include/asm/cache.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES arch/parisc/include/asm/cache.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES arch/powerpc/include/asm/page_32.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES arch/sh/include/asm/page.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES arch/unicore32/include/asm/cache.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES arch/xtensa/include/asm/cache.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES
Hmmm, how does arch/x86 do it?
Regards.
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