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Date: Mon, 17 Nov 2008 22:59:50 +0100 From: Ingo Molnar <mingo@...e.hu> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Eric Dumazet <dada1@...mosbay.com>, David Miller <davem@...emloft.net>, rjw@...k.pl, linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org, cl@...ux-foundation.org, efault@....de, a.p.zijlstra@...llo.nl, Stephen Hemminger <shemminger@...tta.com>, "H. Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de> Subject: system_call() - Re: [Bug #11308] tbench regression on each kernel release from 2.6.22 -> 2.6.28 * Ingo Molnar <mingo@...e.hu> wrote: > 100.000000 total > ................ > 1.508888 system_call that's an easy one: ffffffff8020be00: 97321 <system_call>: ffffffff8020be00: 97321 0f 01 f8 swapgs ffffffff8020be03: 53089 66 66 66 90 xchg %ax,%ax ffffffff8020be07: 1524 66 66 90 xchg %ax,%ax ffffffff8020be0a: 0 66 66 90 xchg %ax,%ax ffffffff8020be0d: 0 66 66 90 xchg %ax,%ax ffffffff8020be10: 1511 <system_call_after_swapgs>: ffffffff8020be10: 1511 65 48 89 24 25 18 00 mov %rsp,%gs:0x18 ffffffff8020be17: 0 00 00 ffffffff8020be19: 0 65 48 8b 24 25 10 00 mov %gs:0x10,%rsp ffffffff8020be20: 0 00 00 ffffffff8020be22: 1490 fb sti syscall entry instruction costs - unavoidable security checks, etc. - hardware costs. But looking at this profile made me notice this detail: ENTRY(system_call_after_swapgs) Combined with this alignment rule we have in arch/x86/include/asm/linkage.h on 64-bit: #ifdef CONFIG_X86_64 #define __ALIGN .p2align 4,,15 #define __ALIGN_STR ".p2align 4,,15" #endif while it inserts NOP sequences, that is still +13 bytes of excessive, stupid, and straight in our syscall entry path alignment padding. system_call_after_swapgs is an utter slowpath in any case. The interim fix is below - although it needs more thinking and probably should be done via an ENTRY_UNALIGNED() method as well, for slowpath targets. With that we get this much nicer entry sequence: ffffffff8020be00: 544323 <system_call>: ffffffff8020be00: 544323 0f 01 f8 swapgs ffffffff8020be03: 197954 <system_call_after_swapgs>: ffffffff8020be03: 197954 65 48 89 24 25 18 00 mov %rsp,%gs:0x18 ffffffff8020be0a: 0 00 00 ffffffff8020be0c: 6578 65 48 8b 24 25 10 00 mov %gs:0x10,%rsp ffffffff8020be13: 0 00 00 ffffffff8020be15: 0 fb sti ffffffff8020be16: 0 48 83 ec 50 sub $0x50,%rsp And we should probably weaken the generic code alignment rules as well on x86. I'll do some measurements of it. Ingo Index: linux/arch/x86/kernel/entry_64.S =================================================================== --- linux.orig/arch/x86/kernel/entry_64.S +++ linux/arch/x86/kernel/entry_64.S @@ -315,7 +315,8 @@ ENTRY(system_call) * after the swapgs, so that it can do the swapgs * for the guest and jump here on syscall. */ -ENTRY(system_call_after_swapgs) +.globl system_call_after_swapgs +system_call_after_swapgs: movq %rsp,%gs:pda_oldrsp movq %gs:pda_kernelstack,%rsp -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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