> -----Original Message----- > From: Will Deacon [mailto:[email protected]] > Sent: Wednesday, May 31, 2017 5:45 AM > To: Pinski, Andrew <[email protected]> > Cc: [email protected]; [email protected]; > Norov, Yuri <[email protected]>; [email protected]; > [email protected]; [email protected]; [email protected]; > [email protected]; [email protected] > Subject: Re: [PATCHv2 1/2] arm64:vdso: Rewrite gettimeofday into C. > > Hi Andrew, > > Thanks for posting this, but please try to cc the maintainers in future -- > I almost missed it!
Oh sorry; I didn't know when I am supposed to CC the maintainers or not. Different project, different rules. > > On Tue, May 30, 2017 at 05:34:19PM -0700, Andrew Pinski wrote: > > This allows the compiler to optimize the divide by 1000. > > And remove the other divide. > > > > On ThunderX, gettimeofday improves by 32%. On ThunderX 2, > > gettimeofday improves by 18%. > > > > Note I noticed a bug in the old implementation of > > __kernel_clock_getres; it was checking only the lower 32bits of the > > pointer; this would work for most cases but could fail in a few. > > > > Changes from v1: > > * Fixed bug in __kernel_clock_getres for checking the pointer argument. > > * Fix comments to refer to functions in arm64. > > I tested this patch on a few platforms I have access to and didn't see the > perf regressions I saw when I looked at this in the past with an older > toolchain (it was mostly about the same, with a couple of improvements). > > So, in principle, I'm not opposed to moving this into C. However, we're > currently close to a "vDSO-explosion" on arm64 with people wanting a compat > variant and also an ILP32 variant. When Kevin posted his compat variant > (also in C): > > http://lkml.kernel.org/r/[email protected] > > Nathan (who apparently needs to set his mail host address ;) was concerned > about duplication between arm and arm64: > > http://lkml.kernel.org/r/[email protected] > address--so-tickle-me > > I'm firmly of the opinion that we should try to write an arch-agnostic vDSO > implementation in core code (lib/vdso or something) where the arch header > provides things like: > > * The mechanism to read the counter > * The mechanism to issue a syscall > * A function to determine whether or not the current clocksource is > suitable > > I think the datapage format could be defined in core code and it would be > worth looking to see how much the virtual mapping code can be consolidated > too. > > If we can get something that works for arm native, arm64 native, arm64 > compat and arm64 ilp32 then it's probably going to be useful for other > architectures too, even if we need to add more customisation points in > future. To share code between the three vdso is a good goal and shouldn't be a hard to expand my patch to handle the arm compat vdso. To expand it to the arm native code shouldn't be too hard. The main thing is add a few #ifdef/#define in a header. I will try to do that but I don't know when I will be able to finish it. Thanks, Andrew > > I've spoken to Kevin about this, but I'm not sure whether he's had a chance > to look at knocking up a prototype. A first stab could just unconditionally > fallback to the system call. > > Will
