From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cdptpa-oedge-vip.email.rr.com (cdptpa-outbound-snat.email.rr.com [107.14.166.226]) by ozlabs.org (Postfix) with ESMTP id 32EEB2C00CB for ; Fri, 18 Oct 2013 04:42:53 +1100 (EST) Date: Thu, 17 Oct 2013 13:35:43 -0400 From: Steven Rostedt To: rkrcmar@redhat.com Subject: Re: [PATCH 5/7] jump_label: relax branch hinting restrictions Message-ID: <20131017133543.7e4e8d45@gandalf.local.home> In-Reply-To: <1382004631-25895-6-git-send-email-rkrcmar@redhat.com> References: <1382004631-25895-1-git-send-email-rkrcmar@redhat.com> <1382004631-25895-6-git-send-email-rkrcmar@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Cc: linux-mips@linux-mips.org, x86@kernel.org, linux-doc@vger.kernel.org, Heiko Carstens , sparclinux@vger.kernel.org, Paul Mackerras , "H. Peter Anvin" , Masami Hiramatsu , linux-s390@vger.kernel.org, Russell King , Raghavendra K T , Ingo Molnar , Andrew Jones , Konrad Rzeszutek Wilk , Thomas Gleixner , linux-arm-kernel@lists.infradead.org, Richard Henderson , Jiri Kosina , linux-kernel@vger.kernel.org, Ralf Baechle , Rob Landley , Martin Schwidefsky , linux390@de.ibm.com, linuxppc-dev@lists.ozlabs.org, "David S. Miller" List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, 17 Oct 2013 12:10:28 +0200 Radim Kr=C4=8Dm=C3=A1=C5=99 wrote: > We implemented the optimized branch selection in higher levels of api. > That made static_keys very unintuitive, so this patch introduces another > element to jump_table, carrying one bit that tells the underlying code > which branch to optimize. >=20 > It is now possible to select optimized branch for every jump_entry. >=20 > Current side effect is 1/3 increase increase in space, we could: > * use bitmasks and selectors on 2+ aligned code/struct. > - aligning jump target is easy, but because it is not done by default > and few bytes in .text are much worse that few kilos in .data, > I chose not to > - data is probably aligned by default on all current architectures, > but programmer can force misalignment of static_key > * optimize each architecture independently > - I can't test everything and this patch shouldn't break anything, so > others can contribute in the future > * choose something worse, like packing or splitting > * ignore >=20 > proof: example & x86_64 disassembly: (F =3D ffffffff) >=20 > struct static_key flexible_feature; > noinline void jump_label_experiment(void) { > if ( static_key_false(&flexible_feature)) > asm ("push 0xa1"); > else asm ("push 0xa0"); > if (!static_key_false(&flexible_feature)) > asm ("push 0xb0"); > else asm ("push 0xb1"); > if ( static_key_true(&flexible_feature)) > asm ("push 0xc1"); > else asm ("push 0xc0"); > if (!static_key_true(&flexible_feature)) > asm ("push 0xd0"); > else asm ("push 0xd1"); > } >=20 > Disassembly of section .text: (push marked by "->") >=20 > F81002000 : > F81002000: e8 7b 29 75 00 callq F81754980 <__fentry__> > F81002005: 55 push %rbp > F81002006: 48 89 e5 mov %rsp,%rbp > F81002009: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) > F8100200e: -> ff 34 25 a0 00 00 00 pushq 0xa0 > F81002015: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) > F8100201a: -> ff 34 25 b0 00 00 00 pushq 0xb0 > F81002021: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) > F81002026: -> ff 34 25 c1 00 00 00 pushq 0xc1 > F8100202d: 0f 1f 00 nopl (%rax) > F81002030: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) > F81002035: -> ff 34 25 d1 00 00 00 pushq 0xd1 > F8100203c: 5d pop %rbp > F8100203d: 0f 1f 00 nopl (%rax) > F81002040: c3 retq This looks exactly like what we want. I take it this is with your patch. What was the result before the patch? -- Steve > F81002041: 0f 1f 80 00 00 00 00 nopl 0x0(%rax) > F81002048: -> ff 34 25 d0 00 00 00 pushq 0xd0 > F8100204f: 5d pop %rbp > F81002050: c3 retq > F81002051: 0f 1f 80 00 00 00 00 nopl 0x0(%rax) > F81002058: -> ff 34 25 c0 00 00 00 pushq 0xc0 > F8100205f: 90 nop > F81002060: eb cb jmp F8100202d <[...]+0x2d> > F81002062: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1) > F81002068: -> ff 34 25 b1 00 00 00 pushq 0xb1 > F8100206f: 90 nop > F81002070: eb af jmp F81002021 <[...]+0x21> > F81002072: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1) > F81002078: -> ff 34 25 a1 00 00 00 pushq 0xa1 > F8100207f: 90 nop > F81002080: eb 93 jmp F81002015 <[...]+0x15> > F81002082: 66 66 66 66 66 2e 0f [...] > F81002089: 1f 84 00 00 00 00 00 >=20 > Contents of section .data: (relevant part of embedded __jump_table)