Re: [PATCH 0/3] Improve ARM atomic performance for malloc

[Will Newton <will dot newton at linaro dot org>]

On 7 October 2014 17:28, Adhemerval Zanella <azanella@linux.vnet.ibm.com> wrote:
> On 07-10-2014 11:18, Torvald Riegel wrote:
>> On Mon, 2014-10-06 at 14:55 +0100, Will Newton wrote:
>>> On 6 October 2014 14:31, Torvald Riegel <triegel@redhat.com> wrote:
>>>> On Fri, 2014-10-03 at 16:27 +0000, Joseph S. Myers wrote:
>>>>> On Fri, 3 Oct 2014, Will Newton wrote:
>>>>>
>>>>>> The resulting atomic.h is hopefully somewhere close to a generic
>>>>>> implementation based on the gcc intrinsics so could potentially
>>>>>> be used as a base for a generic header.
>>>>> That suggests to me that the starting point should be setting up a generic
>>>>> header that can be used for multiple architectures and making the ARM
>>>>> header inherit from it in the case where the relevant compiler support is
>>>>> available, rather than putting all this generic code in an ARM header.
>>>>> (And in turn, the starting point in the generic header could be the
>>>>> particular operations for which more or less generic code already exists
>>>>> in the ARM header, with other operations added to it later.)
>>>> I agree.
>>>>
>>>> In addition, I think that the best step to do this would be when we
>>>> switch to C11-like atomics because with this switch, this falls out kind
>>>> of naturally.
>>>>
>>>> Will, have you looked at my suggestion and the POC patch I posted for
>>>> how C11-like atomics could look like?  I won't get to continue to work
>>>> on this topic this week, but it's still on my agenda.
>>> It's interesting, and long term seems like the best way of doing
>>> things. However I do not see any viable chance of that work being
>>> completed for 2.21. Do you have a timescale in mind? It seems we would
>>> need to convert all uses of the atomic API and all the architecture
>>> ports.
>> I think 2.21 may be fully doable for at least a subset of this.  As I
>> outlined in my other email where I proposed the transition, we indeed do
>> have a big first step in that we need for all architectures to provide
>> C11-like atomics.  I've already scanned through existing code, and I
>> haven't seen any big issues wrt. that: x86 is clear, ARM already uses
>> GCC builtins, for PowerPC we have a clear mapping from C11 to HW
>> instructions.  Many of the "smaller" archs just have simple ops, so
>> there's less specific stuff to do.
>
> I am in favor of support this transition, since I do also also to push the very
> single-thread optimization for PPC. What do you have in mind for the subset of
> this besides your initial approach some weeks ago?
>
> I will try to summarize the topics raised in your thread "Transition to C11
> atomics and memory model" on a wiki entry for 2.21 [1].
>
> [1] https://sourceware.org/glibc/wiki/Release/2.21

Did this information ever get added to the wiki anywhere?

I would like to get consensus on what we can get done for 2.21. I will
respin my patches based on the feedback received so far but I'm also
perfectly happy to help with the C11 atomics work once the direction
is clear.

Thanks,

-- 
Will Newton
Toolchain Working Group, Linaro