From: Tyler Retzlaff <roretzla@linux.microsoft.com>
To: David Marchand <david.marchand@redhat.com>
Cc: dev@dpdk.org, Honnappa.Nagarahalli@arm.com, Ruifeng.Wang@arm.com,
thomas@monjalon.net, Maxime Coquelin <maxime.coquelin@redhat.com>
Subject: Re: [PATCH 0/3] use C11 memory model GCC builtin atomics
Date: Thu, 1 Jun 2023 21:18:14 -0700 [thread overview]
Message-ID: <20230602041814.GA27303@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> (raw)
In-Reply-To: <20230524160508.GA9733@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
On Wed, May 24, 2023 at 09:05:08AM -0700, Tyler Retzlaff wrote:
> 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.
Anything further here or want to keep it as is?
>
> Thanks!
>
> >
> > --
> > David Marchand
next prev parent reply other threads:[~2023-06-02 4:18 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-27 14:30 Tyler Retzlaff
2023-03-27 14:30 ` [PATCH 1/3] bus/vmbus: " Tyler Retzlaff
2023-03-27 14:30 ` [PATCH 2/3] crypto/ccp: " Tyler Retzlaff
2023-03-27 14:30 ` [PATCH 3/3] eal: " Tyler Retzlaff
2023-03-27 15:25 ` [PATCH 0/3] " Morten Brørup
2023-05-24 12:51 ` David Marchand
2023-05-24 16:05 ` Tyler Retzlaff
2023-06-02 4:18 ` Tyler Retzlaff [this message]
2023-06-07 16:36 ` David Marchand
2023-06-07 16:46 ` David Marchand
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