Re: [LKP] [lkp] [x86/hweight] 65ea11ec6a: will-it-scale.per_process_ops 9.3% improvement

["H. Peter Anvin" <hpa@zytor.com>]

On August 25, 2016 2:22:14 AM PDT, Borislav Petkov <bp@suse.de> wrote:
>On Thu, Aug 18, 2016 at 06:11:39AM +0200, Borislav Petkov wrote:
>> So if there's no bug, alternatives should replace all "call
>> __sw_hweightXX" calls with POPCNT. So you shouldn't be even calling
>> these functions and hitting that path.
>> 
>> Can you boot the kernel with "debug-alternative" and put that dmesg
>> somewhere along with vmlinux for me to stare at? Privately is fine
>too.
>> 
>> I'd like to make sure the alternatives application actually happens.
>
>Ok, Huang sent me the files I asked for privately (Thanks!). And I
>still can't
>see how that commit can even influence anything as the code doesn't get
>executed after alternatives:
>
>ffffffff81007f35:       e8 36 66 47 00          callq  ffffffff8147e570
><__sw_hweight64>
>ffffffff81007f35: final_insn: f3 48 0f b8 c7 (popcnt %rdi,%rax)
>
>ffffffff81008021:       e8 4a 65 47 00          callq  ffffffff8147e570
><__sw_hweight64>
>ffffffff81008021: final_insn: f3 48 0f b8 c7 (popcnt %rdi,%rax)
>
>ffffffff8100bd63:       e8 08 28 47 00          callq  ffffffff8147e570
><__sw_hweight64>
>ffffffff8100bd63: final_insn: f3 48 0f b8 c7 (popcnt %rdi,%rax)
>
>ffffffff81171a05:       e8 66 cb 30 00          callq  ffffffff8147e570
><__sw_hweight64>
>ffffffff81171a05: final_insn: f3 48 0f b8 c7 (popcnt %rdi,%rax)
>
>ffffffff81171a66:       e8 05 cb 30 00          callq  ffffffff8147e570
><__sw_hweight64>
>ffffffff81171a66: final_insn: f3 48 0f b8 c7 (popcnt %rdi,%rax)
>
>ffffffff8145c3e5:       e8 86 21 02 00          callq  ffffffff8147e570
><__sw_hweight64>
>ffffffff8145c3e5: final_insn: f3 48 0f b8 c7 (popcnt %rdi,%rax)
>
>ffffffff8145c40c:       e8 5f 21 02 00          callq  ffffffff8147e570
><__sw_hweight64>
>ffffffff8145c40c: final_insn: f3 48 0f b8 c7 (popcnt %rdi,%rax)
>
>ffffffff8174768d:       e8 de 6e d3 ff          callq  ffffffff8147e570
><__sw_hweight64>
>ffffffff8174768d: final_insn: f3 48 0f b8 c7 (popcnt %rdi,%rax)
>
>ffffffff817c43da:       e8 91 a1 cb ff          callq  ffffffff8147e570
><__sw_hweight64>
>ffffffff817c43da: final_insn: f3 48 0f b8 c7 (popcnt %rdi,%rax)
>
>ffffffff817f4e6a:       e8 01 97 c8 ff          callq  ffffffff8147e570
><__sw_hweight64>
>ffffffff817f4e6a: final_insn: f3 48 0f b8 c7 (popcnt %rdi,%rax)
>
>ffffffff81ffae4b:       e8 20 37 48 ff          callq  ffffffff8147e570
><__sw_hweight64>
>ffffffff81ffae4b: final_insn: f3 48 0f b8 c7 (popcnt %rdi,%rax)
>
>ffffffff82011bd1:       e8 9a c9 46 ff          callq  ffffffff8147e570
><__sw_hweight64>
>ffffffff82011bd1: final_insn: f3 48 0f b8 c7 (popcnt %rdi,%rax)
>
>__sw_hweight64 is at 0xffffffff8147e570 and all those locations which
>call 0xffffffff8147e570 get replaced with POPCNT (final_insn in dmesg).
>
>Also, I did this to a guest kernel:
>
>---
>diff --git a/arch/x86/lib/hweight.S b/arch/x86/lib/hweight.S
>index 8a602a1e404a..7f18f59eadd5 100644
>--- a/arch/x86/lib/hweight.S
>+++ b/arch/x86/lib/hweight.S
>@@ -34,6 +34,7 @@ ENTRY(__sw_hweight32)
> ENDPROC(__sw_hweight32)
> 
> ENTRY(__sw_hweight64)
>+	call dump_stack
> #ifdef CONFIG_X86_64
> 	pushq   %rdi
> 	pushq   %rdx
>---
>
>and got 23 invocations before alternatives get applied:
>
>$ grep dump_stack ~/kvm/test-x86_64-1235.log | uniq -c
>     23 [<ffffffff81336955>] dump_stack+0x67/0x92
>
>just to make sure that __sw_hweight64 *actually* *really* gets
>replaced.
>
>Then I ran the job.yaml thing as suggested in the initial mail and no
>more __sw_hweight64 calls.
>
>So either I'm still missing something or that's the wrong commit or ...
>
>/me haz no idea :-\

I'm wondering if one of those 23 invocations sets up some kind of corrupt data that continues to get used.
-- 
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