[dpdk-dev] [PATCH v8 2/6] eal: add the APIs to wait until equal

[David Marchand]

On Tue, Oct 22, 2019 at 11:37 AM Ananyev, Konstantin
<konstantin.ananyev at intel.com> wrote:
>
> > > The rte_wait_until_equal_xx APIs abstract the functionality of
> > > 'polling for a memory location to become equal to a given value'.
> > >
> > > Add the RTE_ARM_USE_WFE configuration entry for aarch64, disabled
> > > by default. When it is enabled, the above APIs will call WFE instruction
> > > to save CPU cycles and power.
> > >
> > > From a VM, when calling this API on aarch64, it may trap in and out to
> > > release vCPUs whereas cause high exit latency. Since kernel 4.18.20 an
> > > adaptive trapping mechanism is introduced to balance the latency and
> > > workload.
> > >
> > > Signed-off-by: Gavin Hu <gavin.hu at arm.com>
> > > Reviewed-by: Ruifeng Wang <ruifeng.wang at arm.com>
> > > Reviewed-by: Steve Capper <steve.capper at arm.com>
> > > Reviewed-by: Ola Liljedahl <ola.liljedahl at arm.com>
> > > Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>
> > > Reviewed-by: Phil Yang <phil.yang at arm.com>
> > > Acked-by: Pavan Nikhilesh <pbhagavatula at marvell.com>
> > > Acked-by: Jerin Jacob <jerinj at marvell.com>
> > > ---
> > >  config/arm/meson.build                             |  1 +
> > >  config/common_base                                 |  5 ++
> > >  .../common/include/arch/arm/rte_pause_64.h         | 26 ++++++
> > >  lib/librte_eal/common/include/generic/rte_pause.h  | 93 ++++++++++++++++++++++
> > >  4 files changed, 125 insertions(+)
> > >
> > > diff --git a/config/arm/meson.build b/config/arm/meson.build
> > > index 979018e..b4b4cac 100644
> > > --- a/config/arm/meson.build
> > > +++ b/config/arm/meson.build
> > > @@ -26,6 +26,7 @@ flags_common_default = [
> > >         ['RTE_LIBRTE_AVP_PMD', false],
> > >
> > >         ['RTE_SCHED_VECTOR', false],
> > > +       ['RTE_ARM_USE_WFE', false],
> > >  ]
> > >
> > >  flags_generic = [
> > > diff --git a/config/common_base b/config/common_base
> > > index e843a21..c812156 100644
> > > --- a/config/common_base
> > > +++ b/config/common_base
> > > @@ -111,6 +111,11 @@ CONFIG_RTE_MAX_VFIO_CONTAINERS=64
> > >  CONFIG_RTE_MALLOC_DEBUG=n
> > >  CONFIG_RTE_EAL_NUMA_AWARE_HUGEPAGES=n
> > >  CONFIG_RTE_USE_LIBBSD=n
> > > +# Use WFE instructions to implement the rte_wait_for_equal_xxx APIs,
> > > +# calling these APIs put the cores in low power state while waiting
> > > +# for the memory address to become equal to the expected value.
> > > +# This is supported only by aarch64.
> > > +CONFIG_RTE_ARM_USE_WFE=n
> > >
> > >  #
> > >  # Recognize/ignore the AVX/AVX512 CPU flags for performance/power testing.
> > > diff --git a/lib/librte_eal/common/include/arch/arm/rte_pause_64.h b/lib/librte_eal/common/include/arch/arm/rte_pause_64.h
> > > index 93895d3..eb8f73e 100644
> > > --- a/lib/librte_eal/common/include/arch/arm/rte_pause_64.h
> > > +++ b/lib/librte_eal/common/include/arch/arm/rte_pause_64.h
> > > @@ -1,5 +1,6 @@
> > >  /* SPDX-License-Identifier: BSD-3-Clause
> > >   * Copyright(c) 2017 Cavium, Inc
> > > + * Copyright(c) 2019 Arm Limited
> > >   */
> >
> > Before including generic/rte_pause.h, put a check like:
> >
> > #ifdef RTE_ARM_USE_WFE
> > #define RTE_ARCH_HAS_WFE
> > #endif
> >
> > #include "generic/rte_pause.h"
> >
> > >
> > >  #ifndef _RTE_PAUSE_ARM64_H_
> > > @@ -17,6 +18,31 @@ static inline void rte_pause(void)
> > >         asm volatile("yield" ::: "memory");
> > >  }
> > >
> > > +#ifdef RTE_ARM_USE_WFE
> > > +#define sev()  { asm volatile("sev" : : : "memory") }
> > > +#define wfe()  { asm volatile("wfe" : : : "memory") }
> > > +
> > > +#define __WAIT_UNTIL_EQUAL(type, size, addr, expected, memorder) \
> > > +__rte_experimental                                             \
> >
> > The experimental tag is unnecessary here.
> > We only need it in the function prototype (in the generic header).
> >
> > > +static __rte_always_inline void                                        \
> > > +rte_wait_until_equal_##size(volatile type * addr, type expected,\
> > > +int memorder)                                                  \
> > > +{                                                              \
> > > +       if (__atomic_load_n(addr, memorder) != expected) {      \
> > > +               sev();                                                  \
> > > +               do {                                                    \
> > > +                       wfe();                                          \
> > > +               } while (__atomic_load_n(addr, memorder) != expected);  \
> > > +        }                                                              \
> > > +}
> > > +__WAIT_UNTIL_EQUAL(uint16_t, 16, addr, expected, memorder)
> > > +__WAIT_UNTIL_EQUAL(uint32_t, 32, addr, expected, memorder)
> > > +__WAIT_UNTIL_EQUAL(uint64_t, 64, addr, expected, memorder)
> > > +
> > > +#undef __WAIT_UNTIL_EQUAL
>
> Might be instead of defining/undefining these macros, just
> define explicitly these 3 functions?
> Now they are really small, so I think it would be an easier/cleaner way.
> Yes, a bit of code duplication, but as I said they are really small now.
> Same thought about generic version.

I don't really like those macros defining inlines either.
I am fine with this little duplication, so +1 from me.


>
> >
> > Missing #undef on sev and wfe macros.
>
> Actually should we undefine them?
> Or should we add rte_ prefix (which is needed anyway I suppose)
> and have them always defined?
> Might be you can reuse them in other arm specific places too (spinlock, rwlock, etc.)
> Actually probably it is possible to make them either emiting a proper instructions or NOP,
> then you'll need RTE_ARM_USE_WFE only around these macros.

Interesting idea, but only if it gets used in this series.
I don't want to see stuff that will not be used later.

>
> I.E
>
> #ifdef RTE_ARM_USE_WFE
> #define rte_sev()  { asm volatile("sev" : : : "memory") }
> #define rte_wfe()  { asm volatile("wfe" : : : "memory") }
> #else
> static inline void rte_sev(void)
> {
> }
> static inline void rte_wfe(void)
> {
>         rte_pause();
> }
> #endif
>
> And then just one common version of _wait_ functios:
>
> static __rte_always_inline void                                        \
> rte_wait_until_equal_32(volatile type * addr, type expected, int memorder)
> {                                                              \
>         if (__atomic_load_n(addr, memorder) != expected) {
>                 rte_sev();
>                 do {
>                                rte_wfe();
>                       } while (__atomic_load_n(addr, memorder) != expected);
>           }
> }
>
>

[snip]

> > Here, change this check to:
> >
> > #ifndef RTE_ARCH_HAS_WFE
> >
> >
> > > +#ifndef RTE_ARM_USE_WFE
>
> Might be something arch neutral in name here?
> RTE_WAIT_UNTIL_EQUAL_ARCH_DEFINED or so?

Yes, better than my suggestion.


Gavin, I noticed that you added a #ifndef ARM in the generic
rte_spinlock.h header later in this series.
Please, can you think of a way to avoid this?
Maybe applying the same pattern to the spinlock?


Thanks.

--
David Marchand