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Subjectsystem_call() - Re: [Bug #11308] tbench regression on each kernel release from 2.6.22 -> 2.6.28

* Ingo Molnar <mingo@elte.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

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