Re: [RFC PATCH 0/5] futex: introduce an optimistic spinning futex

Darren Hart
2014-07-21T17:46:33Z

On 7/21/14, 10:20, "Darren Hart" <dvhart@linux.intel.com> wrote:

>On 7/21/14, 9:45, "Andi Kleen" <andi@firstfloor.org> wrote:
>
>>Andi Kleen <andi@firstfloor.org> writes:
>>
>>> Waiman Long <Waiman.Long@hp.com> writes:
>>>
>>>> This patch series introduces two new futex command codes to support
>>>> a new optimistic spinning futex for implementing an exclusive lock
>>>> primitive that should perform better than the same primitive using
>>>> the wait-wake futex in cases where the lock owner is actively working
>>>> instead of waiting for I/O completion.
>>>
>>> How would you distinguish those two cases automatically?
>>
>>Also BTW traditionally the spinning was just done in user space.
>>
>>This would be always even more efficient, because it would
>>even avoid the syscall entry path.
>>
>>Given the glibc adaptive mutexes are not very good, but I'm sure those
>>could be improved.
>
>I presented on something along these lines a few years back, and various
>people have asked for the sample code over the years. Andi is right,
>doing
>this from user-space has the potential to be more efficient, the
>challenge
>is getting access to privileged information, such as the state of the
>mutex owner. You don't want to spin if the owner is sleeping. I did this
>by adding a tidrunning call to the vdso. My somewhat hacked solution is
>still available here:
>
>http://git.infradead.org/users/dvhart/linux.git/shortlog/refs/heads/futex/
>t
>idrunning/dev
>
>
>I abandoned the spinning in the kernel thing due to the overhead of the
>system call if I remember correctly. Also available here:
>
>http://git.infradead.org/users/dvhart/linux.git/shortlog/refs/heads/futex/
>f
>utex-lock-adaptive
>

And the associated Linux Plumbers 2010 presentation which I can't seem to
find archived anywhere else:

http://dvhart.com/darren/files/kernel-assisted-userspace-adaptive-mutexes.p
df

We observed some significant improvements under some very specific use
cases, but a more thorough dive into performance impact in the other cases
as well as security implications with the vdso is still wanting.

--
Darren Hart Open Source Technology Center
darren.hart@intel.com Intel Corporation