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Date: Tue, 23 Sep 2008 10:50:27 +0200
From: Ingo Molnar <mingo@...e.hu>
To: Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH -tip 0/4] x86: signal handler improvement
* Hiroshi Shimamoto <h-shimamoto@...jp.nec.com> wrote:
> This patch series is experimental.
> I made it against tip/master.
>
> I noticed there are inefficient codes in x86 signals.
> For example, disassembled 32-bit setup_sigcontext();
>
> 0000007c <setup_sigcontext>:
> 7c: 55 push %ebp
> 7d: 89 e5 mov %esp,%ebp
> 7f: 57 push %edi
> 80: 31 ff xor %edi,%edi
> 82: 56 push %esi
> 83: 89 c6 mov %eax,%esi
> 85: 53 push %ebx
> 86: 83 ec 58 sub $0x58,%esp
> 89: 89 55 a4 mov %edx,-0x5c(%ebp)
> 8c: 89 fa mov %edi,%edx
> 8e: 8b 41 24 mov 0x24(%ecx),%eax
> 91: 89 46 04 mov %eax,0x4(%esi)
> 94: 89 55 a8 mov %edx,-0x58(%ebp)
> ...
> 184: 8b 5d ac mov -0x54(%ebp),%ebx
> 187: 0b 5d a8 or -0x58(%ebp),%ebx
> 18a: 0b 5d b0 or -0x50(%ebp),%ebx
> 18d: 0b 5d b4 or -0x4c(%ebp),%ebx
> 190: 0b 5d b8 or -0x48(%ebp),%ebx
> 193: 0b 5d bc or -0x44(%ebp),%ebx
> 196: 0b 5d c0 or -0x40(%ebp),%ebx
> 199: 0b 5d c4 or -0x3c(%ebp),%ebx
> 19c: 0b 5d c8 or -0x38(%ebp),%ebx
> 19f: 0b 5d cc or -0x34(%ebp),%ebx
> 1a2: 0b 5d d0 or -0x30(%ebp),%ebx
> 1a5: 0b 5d d4 or -0x2c(%ebp),%ebx
> 1a8: 0b 5d d8 or -0x28(%ebp),%ebx
> 1ab: 0b 5d dc or -0x24(%ebp),%ebx
> 1ae: 0b 5d e0 or -0x20(%ebp),%ebx
> 1b1: 0b 5d e4 or -0x1c(%ebp),%ebx
> 1b4: 0b 5d e8 or -0x18(%ebp),%ebx
> 1b7: 0b 5d ec or -0x14(%ebp),%ebx
> 1ba: 0b 5d f0 or -0x10(%ebp),%ebx
> 1bd: 09 fb or %edi,%ebx
> ...
> 1dc: 09 d8 or %ebx,%eax
> 1de: 5b pop %ebx
> 1df: 09 c8 or %ecx,%eax
> 1e1: 5e pop %esi
> 1e2: 5f pop %edi
> 1e3: 5d pop %ebp
> 1e4: c3 ret
>
> there is a lot of "or" operation with stack, and it came from a set of
> following lines;
>
> err |= __put_user(x, ptr);
>
> The above line compiled to like this;
> a0: 89 fa mov %edi,%edx
> a2: 8b 41 20 mov 0x20(%ecx),%eax
> a5: 89 46 08 mov %eax,0x8(%esi)
> a8: 89 55 b0 mov %edx,-0x50(%ebp)
>
> and errors in __put_user is stored to stack. At the end of function,
> all errors in stack are calculated. If access to user space is failed,
> %edx is set to -EFAULT in exception handler and stored to stack for
> later operation. It seems inefficient.
yes, very much! Years ago i tried years to improve it but didnt think of
your solution back then.
> This patch series introduce __{put|get}_user_cerr for cumulative error
> handling. So, the line;
> err |= __put_user(x, ptr);
> changes to
> __put_user_cerr(x, ptr, err);
>
> and it compiled to like this;
> 92: 8b 41 20 mov 0x20(%ecx),%eax
> 95: 89 47 08 mov %eax,0x8(%edi)
>
> The cumulative error is kept in the other register, in this example,
> %esi is used for this, and returns it. If access to user space causes fault,
> %esi is set to the value (%esi | -EFAULT) in exception handler.
>
> 137: 89 f0 mov %esi,%eax
> 139: 5b pop %ebx
> 13a: 5e pop %esi
> 13b: 5f pop %edi
> 13c: 5d pop %ebp
> 13d: c3 ret
>
> The results of this, I got a little performance improvement in signal
> handling. Here is a result of lmbench.
> I've tried 64-bit only at this time. Will measure on 32-bit.
>
> Processor, Processes - times in microseconds - smaller is better
> ------------------------------------------------------------------------------
> Host OS Mhz null null open slct sig sig fork exec sh
> call I/O stat clos TCP inst hndl proc proc proc
> --------- ------------- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----
> tip64-sig Linux 2.6.27- 3788 0.17 0.27 1.67 2.65 3.20 0.32 2.97 180. 524. 1806
> tip64-sig Linux 2.6.27- 3788 0.17 0.27 1.68 2.65 3.20 0.32 2.95 180. 520. 1796
> tip64-sig Linux 2.6.27- 3788 0.17 0.27 1.68 2.72 3.21 0.32 2.96 177. 528. 1812
> tip64 Linux 2.6.27- 3788 0.17 0.27 1.68 2.74 3.22 0.32 3.01 174. 523. 1853
> tip64 Linux 2.6.27- 3788 0.17 0.27 1.68 2.73 3.22 0.32 3.01 175. 523. 1806
> tip64 Linux 2.6.27- 3788 0.17 0.27 1.68 2.74 3.20 0.32 3.01 181. 523. 1791
>
> This patch series also reduces stack usages and code size.
>
> $ size signal_*
> text data bss dec hex filename
> 4507 0 0 4507 119b signal_32.o.new
> 5031 0 0 5031 13a7 signal_32.o.old
> 3827 0 0 3827 ef3 signal_64.o.new
> 4652 0 0 4652 122c signal_64.o.old
>
> Comments are welcome.
> I'll handle this patch series if it's good.
very nice!!
could we perhaps first finish unifying them into signal.c, and then
introduce __put_user_cerr() in signal_32/64.c?
Or we could put your patches into tip/x86/signal as-is if you expect to
finish the unification in the near future.
Ingo
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