On Thu, May 26, 2016 at 11:48:19PM +0300, Yury Norov wrote: > On Wed, May 25, 2016 at 02:28:21PM -0700, David Miller wrote: > > From: Arnd Bergmann <arnd@xxxxxxxx> > > Date: Wed, 25 May 2016 23:01:06 +0200 > > > > > On Wednesday, May 25, 2016 1:50:39 PM CEST David Miller wrote: > > >> From: Arnd Bergmann <arnd@xxxxxxxx> > > >> Date: Wed, 25 May 2016 22:47:33 +0200 > > >> > > >> > If we use the normal calling conventions, we could remove these overrides > > >> > along with the respective special-case handling in glibc. None of them > > >> > look particularly performance-sensitive, but I could be wrong there. > > >> > > >> You could set the lowest bit in the system call entry pointer to indicate > > >> the upper-half clears should be elided. > > > > > > Right, but that would introduce an extra conditional branch in the syscall > > > hotpath, and likely eliminate the gains from passing the loff_t arguments > > > in a single register instead of a pair. > > > > Ok, then, how much are you really gaining from avoiding a 'shift' and > > an 'or' to build the full 64-bit value? 3 cycles? Maybe 4? > > 4 cycles in kernel and ~same cost in glibc to create a pair. It would take a single instruction per argument in the kernel to do shift+or and maybe 1-2 more instructions to move the remaining arguments in place (we do this for a few wrappers in arch/arm64/kernel/entry32.S). And the glibc counterpart. > And 8 'mov's that exist for every syscall, even yield(). > > > And the executing the wrappers, those have a non-trivial cost too. > > The cost is pretty trivial though. See kernel/compat_wrapper.o: > COMPAT_SYSCALL_WRAP2(creat, const char __user *, pathname, umode_t, mode); > 0: a9bf7bfd stp x29, x30, [sp,#-16]! > 4: 910003fd mov x29, sp > 8: 2a0003e0 mov w0, w0 > c: 94000000 bl 0 <sys_creat> > 10: a8c17bfd ldp x29, x30, [sp],#16 > 14: d65f03c0 ret I would say the above could be more expensive than 8 movs (16 bytes to write, read, a branch and a ret). You can also add the I-cache locality, having wrappers for each syscalls instead of a single place for zeroing the upper half (where no other wrapper is necessary). Can we trick the compiler into doing a tail call optimisation. This could have simply been: COMPAT_SYSCALL_WRAP2(creat, ...): mov w0, w0 b <sys_creat> > > Cost wise, this seems like it all cancels out in the end, but what > > do I know? > > I think you know something, and I also think Heiko and other s390 guys > know something as well. So I'd like to listen their arguments here. > > For me spark64 way is looking reasonable only because it's really simple > and takes less coding. I'll try it on some branch and share here what happened. The kernel code will definitely look simpler ;). It would be good to see if there actually is any performance impact. Even with 16 more cycles on syscall entry, would they be lost in the noise? You don't need a full implementation, just some dummy mov x0, x0 on the entry path. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
