Re: [dpdk-dev] [PATCH v3 5/5] config: add WFE config entry for aarch64

["Gavin Hu (Arm Technology China)" <Gavin.Hu@arm.com>]

Hi Stephen,
> -----Original Message-----
> From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> Sent: Wednesday, July 24, 2019 3:10 AM
> To: Stephen Hemminger <stephen@networkplumber.org>; Gavin Hu (Arm
> Technology China) <Gavin.Hu@arm.com>
> Cc: dev@dpdk.org; nd <nd@arm.com>; thomas@monjalon.net;
> jerinj@marvell.com; pbhagavatula@marvell.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; nd <nd@arm.com>
> Subject: RE: [PATCH v3 5/5] config: add WFE config entry for aarch64
> 
> >
> > On Tue, 23 Jul 2019 23:43:46 +0800
> > Gavin Hu <gavin.hu@arm.com> wrote:
> >
> > > Add the RTE_USE_WFE configuration entry for aarch64, disabled by
> default.
> > > It can be enabled selectively based on the performance benchmarking.
> > >
> > > Signed-off-by: Gavin Hu <gavin.hu@arm.com>
> > > Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > > Reviewed-by: Steve Capper <steve.capper@arm.com>
> > > Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> > > Acked-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
> > > ---
> > >  config/arm/meson.build     | 1 +
> > >  config/common_armv8a_linux | 6 ++++++
> > >  2 files changed, 7 insertions(+)
> > >
> > > diff --git a/config/arm/meson.build b/config/arm/meson.build index
> > > 979018e..496813a 100644
> > > --- a/config/arm/meson.build
> > > +++ b/config/arm/meson.build
> > > @@ -116,6 +116,7 @@ impl_dpaa = ['NXP DPAA', flags_dpaa,
> > > machine_args_generic]
> > >  impl_dpaa2 = ['NXP DPAA2', flags_dpaa2, machine_args_generic]
> > >
> > >  dpdk_conf.set('RTE_FORCE_INTRINSICS', 1)
> > > +dpdk_conf.set('RTE_USE_WFE', 0)
> > >
> > >  if not dpdk_conf.get('RTE_ARCH_64')
> > >  dpdk_conf.set('RTE_CACHE_LINE_SIZE', 64) diff --git
> > > a/config/common_armv8a_linux b/config/common_armv8a_linux index
> > > 481712e..48c7ab5 100644
> > > --- a/config/common_armv8a_linux
> > > +++ b/config/common_armv8a_linux
> > > @@ -12,6 +12,12 @@ CONFIG_RTE_ARCH_64=y
> > >
> > >  CONFIG_RTE_FORCE_INTRINSICS=y
> > >
> > > +# Use WFE instructions to implement the rte_wait_for_equal_xxx APIs,
> > > +# calling these APIs put the cores enter low power state while
> > > +waiting # for the memory address to be become equal to the expected
> value.
> > > +# This is supported only by aarch64.
> > > +CONFIG_RTE_USE_WFE=n
> > > +
> > >  # Maximum available cache line size in arm64 implementations.
> > >  # Setting to maximum available cache line size in generic config  #
> > > to address minimum DMA alignment across all arm64 implementations.
> >
> > Introducing config options is a maintenance nightmare.
> > How are distributions supposed to ship a package?
> > Does full regression test get done on both options?
> >
> > The user should not be able to change this.
> Agree with these concerns here. In our tests, we are finding that this patch
> does not result in performance improvements on all micro-architectures.
> May be these micro-architectures will evolve in the future knowing that
> WFE is being used in DPDK. But at this point, it does not make sense to
> enable this by default. This means additional testing/regression with the flag
> enabled. We could add this to Travis build (Travis yml file).
> 
> Currently, this patch will address use cases where the target
> hardware/environment is known during compilation.
In our testing, like running testpmd and packet_ordering(for WFE ring benchmarking), it showed no improvements nor degradation in performance. 
For some micro-benchmarking, it showed slight improvements sometimes, no degradation were seen.
The added benefit of the patch set is power saving, but it is not a primary concern in DPDK and we are short of measurement ways to benchmark that.