On 07/21/2015 02:15 PM, Borislav Petkov wrote: > On Tue, Jul 21, 2015 at 06:12:15PM +0200, Peter Zijlstra wrote: >> Yes, if you start out false, you must be unlikely. If you start out >> true, you must be likely. >> >> We could maybe try and untangle that if there really is a good use case, >> but this is the current state. >> >> The whole reason this happened is because 'false' is like: >> >> >> ... >> <nop> >> 1: >> ... >> >> >> >> label: >> <unlikely code> >> jmp 1b >> >> >> Where the code if out-of-line by default. The enable will rewrite the >> <nop> with a jmp label. > Btw, native_sched_clock() is kinda botched because of that, see below. > > I'd want that RDTSC to come first with a NOP preceding it which can > become a JMP in case some idiotic CPU can't do RDTSC and needs to use > jiffies. Instead, we *unconditionally* jump to RDTSC which is WTF?! We > can just as well do a normal unlikely() without the static_key:
hmmm...so this is a case where need to the default the branch to the out-of-line branch at boot. That is, we can't just enable the out-of-line branch at boot time, b/c it might be too late at that point? IE native_sched_clock() gets called very early? Thanks, -Jason > > .globl native_sched_clock > .type native_sched_clock, @function > native_sched_clock: > pushq %rbp # > movq %rsp, %rbp #, > #APP > # 21 "./arch/x86/include/asm/jump_label.h" 1 > 1:.byte 0x0f,0x1f,0x44,0x00,0 > .pushsection __jump_table, "aw" > .balign 8 > .quad 1b, .L122, __use_tsc #, > .popsection > > # 0 "" 2 > #NO_APP > movabsq $-4294667296000000, %rax #, tmp118 > popq %rbp # > imulq $1000000, jiffies_64(%rip), %rdx #, jiffies_64, D.28443 > addq %rdx, %rax # D.28443, D.28443 > ret > .L122: > #APP > # 118 "./arch/x86/include/asm/msr.h" 1 > rdtsc > # 0 "" 2 > -- 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/
