Re: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race withpreemptible kernel and CPU hotplug

[Mathieu Desnoyers]

* Jeremy Fitzhardinge (jeremy@xxxxxxxx) wrote:
> Mathieu Desnoyers wrote:
>> * H. Peter Anvin (hpa@xxxxxxxxx) wrote:
>>   
>>> Mathieu Desnoyers wrote:
>>>     
>>>> I can't argue about the benefit of using VM CPU pinning to manage
>>>> resources because I don't use it myself, but I ran some tests out of
>>>> curiosity to find if uncontended locks were that cheap, and it turns out
>>>> they aren't. Here are the results :
>>>> Xeon 2.0GHz
>>>> Summary
>>>> make -j1 kernel/      33.94 +/- 0.07         34.91 +/- 0.27      2.8 %
>>>> hackbench 50           2.99 +/- 0.01          3.74 +/- 0.01     25.1 %
>>>> 1 CPU, replace smp lock prefixes with DS segment selector prefixes
>>>> 1 CPU, noreplace-smp
>>>>       
>>> For reference, could you also compare replace smp lock with NOPs?
>>>
>>> 	-hpa
>>>     
>>
>> Sure, here are the updated tables. Basically, they show no significant
>> difference between the NOP and the DS segment selector prefix
>> approaches.
>>   
>
> BTW, are you changing the initial prefix to DS too?  Ie, are you doing a 
> nop->lock->ds transition, or ds->lock->ds?
>
>    J

Yeah, I thought about this case yesterday, good thing you ask.

include/asm-x86/alternative.h defines LOCK_PREFIX as :

#define LOCK_PREFIX \
                ".section .smp_locks,\"a\"\n"   \
                _ASM_ALIGN "\n"                 \
                _ASM_PTR "661f\n" /* address */ \
                ".previous\n"                   \
                "661:\n\tlock; "

So we have the locked instructions built into the kernel, not the nop'd
one. Therefore, the only transition I am doing for my benchmarks is :

lock->ds

but I tried to switch back to SMP and it worked fine.

Mathieu


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Mathieu Desnoyers
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