Re: [PATCH 0/3] use C11 memory model GCC builtin atomics

[Tyler Retzlaff <roretzla@linux.microsoft.com>]

On Wed, May 24, 2023 at 02:51:50PM +0200, David Marchand wrote:
> On Mon, Mar 27, 2023 at 4:30 PM Tyler Retzlaff
> <roretzla@linux.microsoft.com> wrote:
> >
> > Replace the use of __sync_<op>_and_fetch and __sync_fetch_and_<op> atomics
> > with GCC C11 memory model __atomic builtins.
> >
> > This series contributes to converging on standard atomics in 23.11 but is
> > kept separate as there may be sensitivity to converting from __sync to the
> > C11 memory model builtins.
> 
> - Looking at the patches, I thought the conversion was rather straightforward.
> But this mention about "sensitivity" stopped me from merging.
> Did I miss some risk with the changes of this series?
> 
> 
> >
> > Tyler Retzlaff (3):
> >   bus/vmbus: use C11 memory model GCC builtin atomics
> >   crypto/ccp: use C11 memory model GCC builtin atomics
> >   eal: use C11 memory model GCC builtin atomics
> >
> >  drivers/bus/vmbus/vmbus_channel.c    |  2 +-
> >  drivers/crypto/ccp/ccp_dev.c         |  6 ++++--
> >  lib/eal/include/generic/rte_atomic.h | 32 ++++++++++++++++----------------
> >  3 files changed, 21 insertions(+), 19 deletions(-)
> 
> 
> - I noticed that the vhost library has been providing an internal
> wrapper for some __sync atomic with older GCC.
> Some details are in the commitlog c16915b87109 ("vhost: improve dirty
> pages logging performance").
> 
> Could it affect the existing legacy API performance?

Yes.

gcc documents that you can replace __sync_<op> with __atomic_<op> using
SEQ_CST ordering.

When the __atomic_<op> builtins were initially introduced they generated
sub-optimal (you can interpret as slower) codegen relative to the
existing __sync_<op> builtins which was fixed in later gcc releases.

I do not know the actual version of gcc, but the commit you reference
indicates GCC_VERSION < 70100 is that boundary.

I (perhaps incorrectly) assumed that if the CI performance tests didn't
indicate a regression that the replacement of the remaining and minimal
use of the legacy API would have negligable impact.

If this is a bad assumption or there are concerns, I could update the series
to do the conditional __sync vs __atomic throughout.

Let me know how you'd like to proceed.

Thanks!

> 
> -- 
> David Marchand