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Date: Tue, 23 Sep 2008 09:58:54 -0700
From: Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
To: Ingo Molnar <mingo@...e.hu>
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
Ingo Molnar wrote:
> * 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.
Thanks for looking this series!
I prefer to finish unification first.
Will push this series after unification.
thanks,
Hiroshi Shimamoto
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