Messages in this thread | | | Subject | Re: [PATCH v1] clk: Convert managed get functions to devm_add_action API | From | Marc Gonzalez <> | Date | Wed, 11 Dec 2019 17:17:28 +0100 |
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On 02/12/2019 14:51, Robin Murphy wrote:
> 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.
Aaah... memory overhead. Thanks for pointing it out.
BEFORE
devm_clk_get() -> devres_alloc(devm_clk_release, sizeof(*ptr), GFP_KERNEL); allocates space for a struct devres + a pointer
struct devres { struct devres_node node; /* * Some archs want to perform DMA into kmalloc caches * and need a guaranteed alignment larger than * the alignment of a 64-bit integer. * Thus we use ARCH_KMALLOC_MINALIGN here and get exactly the same * buffer alignment as if it was allocated by plain kmalloc(). */ u8 __aligned(ARCH_KMALLOC_MINALIGN) data[]; };
Not sure what it means for a flexible array member to be X-aligned...
(Since the field's address depends on the start address, which is only determined at run-time...)
For example, on arm64, ARCH_KMALLOC_MINALIGN appears to be 128 (sometimes).
/* * Memory returned by kmalloc() may be used for DMA, so we must make * sure that all such allocations are cache aligned. Otherwise, * unrelated code may cause parts of the buffer to be read into the * cache before the transfer is done, causing old data to be seen by * the CPU. */ #define ARCH_DMA_MINALIGN (128)
Unless the strict alignment is also imposed on kmalloc?
So basically, a struct devres starts on a multiple-of-128 address, first the devres_node member, then padding to the next 128, then the data member?
/* * Some archs want to perform DMA into kmalloc caches and need a guaranteed * alignment larger than the alignment of a 64-bit integer. * Setting ARCH_KMALLOC_MINALIGN in arch headers allows that. */ #if defined(ARCH_DMA_MINALIGN) && ARCH_DMA_MINALIGN > 8 #define ARCH_KMALLOC_MINALIGN ARCH_DMA_MINALIGN #define KMALLOC_MIN_SIZE ARCH_DMA_MINALIGN #define KMALLOC_SHIFT_LOW ilog2(ARCH_DMA_MINALIGN) #else #define ARCH_KMALLOC_MINALIGN __alignof__(unsigned long long) #endif
A devres_node boils down to 2 object pointers + 1 function pointer.
Are there architectures supported by Linux where a function pointer is not the same size as an object pointer? (ia64 maybe?)
OK, I will give this patch some more thought.
But I need to ask: what is the rationale for the devm_add_action API?
Regards.
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