linux-kernel - Re: [PATCH] futex: bugfix for futex-key conflict when futex use hugepage

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Date:	Thu, 18 Apr 2013 07:34:29 -0700
From:	Darren Hart <dvhart@...ux.intel.com>
To:	zhang.yi20@....com.cn
CC:	Dave Hansen <dave@...ux.vnet.ibm.com>, Dave Hansen <dave@...1.net>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] futex: bugfix for futex-key conflict when futex use hugepage



On 04/18/2013 01:05 AM, zhang.yi20@....com.cn wrote:
> Darren Hart <dvhart@...ux.intel.com> wrote on 2013/04/17 23:51:36:
> 
>> On 04/17/2013 08:26 AM, Dave Hansen wrote:
>>> On 04/17/2013 07:18 AM, Darren Hart wrote:
>>>>>> This also needs a comment in futex.h describing the usage of the
>>>>>> offset field in union futex_key as well as above get_futex_key
>>>>>> describing the key for shared mappings.
>>>>>>
>>>>> As far as I know , the max size of one hugepage is 1 GBytes for 
>>>>> x86 cpu. Can some other cpus support greater hugepage even more 
>>>>> than 4 GBytes? If so, we can change the type of 'offset' from int 
>>>>>  to long to avoid truncating.
>>>>
>>>> I discussed this with Dave Hansen, on CC, and he thought we needed
>>>> 9 bits, so even on x86 32b we should be covered.
>>>
>>> I think the problem is actually on 64-bit since you still only have
>>> 32-bits in an 'int' there.
>>>
>>> I guess it's remotely possible that we could have some
>>> mega-super-huge-gigantic pages show up in hardware some day, or that
>>> somebody would come up with software-only one.  I bet there's a lot
>>> more code that will break in the kernel than this futex code, though.
>>>
>>> The other option would be to start #defining some build-time constant
>>> for what the largest possible huge page size is, then BUILD_BUG_ON()
>>> it.
>>>
>>> Or you can just make it a long ;)
>>
>> If we make it a long I'd want to see futextest performance tests before
>> and after. Messing with the futex_key has been known to have bad results
>> in the past :-)
>>
>> -- 
>  
> I have run futextest/performance/futex_wait for testing, 5 times before 
> make it long:
> futex_wait: Measure FUTEX_WAIT operations per second
>         Arguments: iterations=100000000 threads=256
> Result: 10215 Kiter/s
> 
> futex_wait: Measure FUTEX_WAIT operations per second
>         Arguments: iterations=100000000 threads=256
> Result: 9862 Kiter/s
> 
> futex_wait: Measure FUTEX_WAIT operations per second
>         Arguments: iterations=100000000 threads=256
> Result: 10081 Kiter/s
> 
> futex_wait: Measure FUTEX_WAIT operations per second
>         Arguments: iterations=100000000 threads=256
> Result: 10060 Kiter/s
> 
> futex_wait: Measure FUTEX_WAIT operations per second
>         Arguments: iterations=100000000 threads=256
> Result: 10081 Kiter/s
> 
> 
> And 5 times after make it long:
> futex_wait: Measure FUTEX_WAIT operations per second
>         Arguments: iterations=100000000 threads=256
> Result: 9940 Kiter/s
> 
> futex_wait: Measure FUTEX_WAIT operations per second
>         Arguments: iterations=100000000 threads=256
> Result: 10204 Kiter/s
> 
> futex_wait: Measure FUTEX_WAIT operations per second
>         Arguments: iterations=100000000 threads=256
> Result: 9901 Kiter/s
> 
> futex_wait: Measure FUTEX_WAIT operations per second
>         Arguments: iterations=100000000 threads=256
> Result: 10152 Kiter/s
> 
> futex_wait: Measure FUTEX_WAIT operations per second
>         Arguments: iterations=100000000 threads=256
> Result: 10060 Kiter/s
> 
> 
> Seems OK, is it?
> 

Changes appear to be in the noise, no impact with this load anyway.

How many CPUs on your test machine? I presume not 256?

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Technical Lead - Linux Kernel
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