> On Dec 3, 2018, at 2:49 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Mon, Dec 03, 2018 at 02:04:41PM -0800, Nadav Amit wrote: >> On Dec 3, 2018, at 8:13 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: >>> On Mon, Dec 03, 2018 at 02:59:36PM +0100, Vlastimil Babka wrote: >>>> On 11/28/18 3:01 PM, Matthew Wilcox wrote: >>>>> Some of the functions in vmalloc.c have as many as nine arguments. >>>>> So I thought I'd have a quick go at bundling the ones that make sense >>>>> into a struct and pass around a pointer to that struct. Well, it made >>>>> the generated code worse, >>>> >>>> Worse in which metric? >>> >>> More instructions to accomplish the same thing. >>> >>>>> so I thought I'd share my attempt so nobody >>>>> else bothers (or soebody points out that I did something stupid). >>>> >>>> I guess in some of the functions the args parameter could be const? >>>> Might make some difference. >>>> >>>> Anyway this shouldn't be a fast path, so even if the generated code is >>>> e.g. somewhat larger, then it still might make sense to reduce the >>>> insane parameter lists. >>> >>> It might ... I'm not sure it's even easier to program than the original >>> though. >> >> My intuition is that if all the fields of vm_args were initialized together >> (in the same function), and a 'const struct vm_args *' was provided as >> an argument to other functions, code would be better (at least better than >> what you got right now). >> >> I’m not saying it is easily applicable in this use-case (since I didn’t >> check). > > Your intuition is wrong ... > > text data bss dec hex filename > 9466 81 32 9579 256b before.o > 9546 81 32 9659 25bb .build-tiny/mm/vmalloc.o > 9546 81 32 9659 25bb const.o > > indeed, there's no difference between with or without the const, according > to 'cmp'. > > Now, only alloc_vmap_area() gets to take a const argument. > __get_vm_area_node() intentionally modifies the arguments. But feel > free to play around with this; you might be able to make it do something > worthwhile. I was playing with it (a bit). What I suggested (modifying __get_vm_area_node() so it will not change arguments) helps a bit, but not much. One insight that I got is that at least part of the overhead comes from the the stack protector code that gcc emits.
