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

[Torvald Riegel <triegel at redhat dot com>]

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.

The C11-like atomics would then coexist for a while with the old-style
atomics.  We can then move one piece of concurrent code (ie, a cluster
of functions that's complete in terms of including all functions that
another function in the cluster synchronizes with) to C11-style atomics
at a time.  There's no hurry to do this before 2.21, although I already
spotted a few things that are likely bugs (and they do affect ARM and
Power).

What I do need though is consensus from the community that the move
towards C11 is fine, and feedback on any patches for that.