On Tue, Mar 10, 2015 at 12:57 AM, Ingo Molnar <mi...@kernel.org> wrote: > > * Denys Vlasenko <dvlas...@redhat.com> wrote: > >> Old code was trying to avoid having three branch insns, >> but instead it has a chain of six insns where each insn >> depends on previos one. >> >> And it was touching PT_OLDSS(%esp) unconditionally, even when it may >> contain bogus data. Elsewhere we have to jump thru hoops >> just to make sure here PT_OLDSS(%esp) is at least in a valid page. >> >> All this just to have one branch instead of three? >> >> The new code simply checks each condition. >> All three checks can run in parallel on an out-of-order CPU. >> Most of the time, none of branches will be taken. >> >> Comparison of object code: >> Old: >> 1e6: 8b 44 24 38 mov 0x38(%esp),%eax >> 1ea: 8a 64 24 40 mov 0x40(%esp),%ah >> 1ee: 8a 44 24 34 mov 0x34(%esp),%al >> 1f2: 25 03 04 02 00 and $0x20403,%eax >> 1f7: 3d 03 04 00 00 cmp $0x403,%eax >> 1fc: 74 0f je 20d <ldt_ss> >> New: >> 1e6: f6 44 24 3a 02 testb $0x2,0x3a(%esp) >> 1eb: 75 0e jne 1fb <restore_nocheck> >> 1ed: f6 44 24 34 03 testb $0x3,0x34(%esp) >> 1f2: 74 07 je 1fb <restore_nocheck> >> 1f4: f6 44 24 40 04 testb $0x4,0x40(%esp) >> 1f9: 75 0f jne 20a <ldt_ss> > > Please do some benchmarking of this: a tight loop of getpid or getppid > syscalls ought to be enough to be able to time this accurately.
Before you benchmark, I think you should reorder it: check CS, then OLDSS, then EFLAGS. The case where CS & 3 == 0, OLDSS & 4 == 4, and EFLAGS & VM == VM should be *extremely* rare. I'm going to hold off on resending my sp0/sp1/ss cleanups until we resolve this -- if it turns out that your code is the same or faster and therefore gets merged, then I think that all the -8 crap can just be deleted instead of being fixed. --Andy > > Thanks, > > Ingo -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
