On Fri, Feb 12, 2016 at 09:10:11PM +0100, Peter Zijlstra wrote: > On Fri, Feb 12, 2016 at 12:32:06PM -0600, Josh Poimboeuf wrote: > > What I actually see in the listing is: > > > > decl __percpu_prefix:__preempt_count > > je 1f: > > .... > > 1: > > call ___preempt_schedule > > > > So it puts the "call ___preempt_schedule" in the slow path. > > Ah yes indeed. Same difference though. > > > I also don't see how that would be related to the use of the asm > > statement in the __preempt_schedule() macro. Doesn't the use of > > unlikely() in preempt_enable() put the call in the slow path? > > Sadly no, unlikely() and asm_goto don't work well together. But the slow > path or not isn't the reason we do the asm call thing. > > > #define preempt_enable() \ > > do { \ > > barrier(); \ > > if (unlikely(preempt_count_dec_and_test())) \ > > preempt_schedule(); \ > > } while (0) > > > > Also, why is the thunk needed? Any reason why preempt_enable() can't be > > called directly from C? > > That would make the call-site save registers and increase the size of > every preempt_enable(). By using the thunk we can do callee saved > registers and avoid blowing up the call site. So is the goal to optimize for size? If I replace the calls to __preempt_schedule[_notrace]() with real C calls and remove the thunks, it only adds about 2k to vmlinux. There are two ways to fix the warnings: 1. get rid of the thunks and call the C functions directly; or 2. add the stack pointer to the asm() statement output operand list to ensure a stack frame gets created in the caller function before the call. (Note this still allows the thunks to do callee saved registers.) I like #1 better, but maybe I'm still missing the point of the thunks. -- Josh -- To unsubscribe from this list: send the line "unsubscribe linux-watchdog" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html