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

Darren Hart
2013-04-18T14:34:31Z

On 04/18/2013 01:05 AM, zhang.yi20@zte.com.cn wrote:
> Darren Hart <dvhart@linux.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