Re: [dpdk-dev] [PATCH 1/1] eal: add 128-bit cmpset (x86-64 only)

["Eads, Gage" <gage.eads@intel.com>]

> -----Original Message-----
> From: Ola Liljedahl [mailto:Ola.Liljedahl@arm.com]
> Sent: Monday, January 28, 2019 5:02 PM
> To: Eads, Gage <gage.eads@intel.com>; dev@dpdk.org
> Cc: arybchenko@solarflare.com; jerinj@marvell.com;
> chaozhu@linux.vnet.ibm.com; nd <nd@arm.com>; Richardson, Bruce
> <bruce.richardson@intel.com>; Ananyev, Konstantin
> <konstantin.ananyev@intel.com>; hemant.agrawal@nxp.com;
> olivier.matz@6wind.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; Gavin Hu (Arm Technology China)
> <Gavin.Hu@arm.com>
> Subject: Re: [dpdk-dev] [PATCH 1/1] eal: add 128-bit cmpset (x86-64 only)
> 
> On Mon, 2019-01-28 at 11:29 -0600, Gage Eads wrote:
> > This operation can be used for non-blocking algorithms, such as a
> > non-blocking stack or ring.
> >
> > Signed-off-by: Gage Eads <gage.eads@intel.com>
> > ---
> >  .../common/include/arch/x86/rte_atomic_64.h        | 31 +++++++++++
> >  lib/librte_eal/common/include/generic/rte_atomic.h | 65
> > ++++++++++++++++++++++
> >  2 files changed, 96 insertions(+)
> >
> > diff --git a/lib/librte_eal/common/include/arch/x86/rte_atomic_64.h
> > b/lib/librte_eal/common/include/arch/x86/rte_atomic_64.h
> > index fd2ec9c53..b7b90b83e 100644
> > --- a/lib/librte_eal/common/include/arch/x86/rte_atomic_64.h
> > +++ b/lib/librte_eal/common/include/arch/x86/rte_atomic_64.h
> > @@ -34,6 +34,7 @@
> >  /*
> >   * Inspired from FreeBSD src/sys/amd64/include/atomic.h
> >   * Copyright (c) 1998 Doug Rabson
> > + * Copyright (c) 2019 Intel Corporation
> >   * All rights reserved.
> >   */
> >
> > @@ -46,6 +47,7 @@
> >
> >  #include <stdint.h>
> >  #include <rte_common.h>
> > +#include <rte_compat.h>
> >  #include <rte_atomic.h>
> >
> >  /*------------------------- 64 bit atomic operations
> > ------------------------ -*/ @@ -208,4 +210,33 @@ static inline void
> > rte_atomic64_clear(rte_atomic64_t *v)
> >  }
> >  #endif
> >
> > +static inline int __rte_experimental
> __rte_always_inline?
> 
> > +rte_atomic128_cmpset(volatile rte_int128_t *dst,
> No need to declare the location volatile. Volatile doesn't do what you think it
> does.
> https://youtu.be/lkgszkPnV8g?t=1027
> 

I made this volatile to match the existing rte_atomicN_cmpset definitions, which presumably have a good reason for using the keyword. Maintainers, any input here?

> 
> > +		     rte_int128_t *exp,
> I would declare 'exp' const as well and document that 'exp' is not updated (with
> the old value) for a failure. The reason being that ARMv8.0/AArch64 cannot
> atomically read the old value without also writing the location and that is bad
> for performance (unnecessary writes leads to unnecessary contention and
> worse scalability). And the user must anyway read the location (in the start of
> the critical section) using e.g. non-atomic 64-bit reads so there isn't actually any
> requirement for an atomic 128-bit read of the location.
> 

Will change in v2.

> >  rte_int128_t *src,
> const rte_int128_t *src?

Sure, I don't see any harm in using const.

> 
> But why are we not passing 'exp' and 'src' by value? That works great, even with
> structs. Passing by value simplifies the compiler's life, especially if the call is
> inlined. Ask a compiler developer.

I ran objdump on the nb_stack code with both approaches, and pass-by-reference resulted in fewer overall x86_64 assembly ops.
PBV: 100 ops for push, 97 ops for pop
PBR: 92 ops for push, 84 ops for pop

(Using the in-progress v5 nb_stack code)

Another factor -- though much less compelling -- is that with pass-by-reference, the user can create a 16B structure and cast it to rte_int128_t when they call rte_atomic128_cmpset, whereas with pass-by-value they need to put that struct in a union with rte_int128_t.

> 
> > +		     unsigned int weak,
> > +		     enum rte_atomic_memmodel_t success,
> > +		     enum rte_atomic_memmodel_t failure) {
> > +	RTE_SET_USED(weak);
> > +	RTE_SET_USED(success);
> > +	RTE_SET_USED(failure);
> > +	uint8_t res;
> > +
> > +	asm volatile (
> > +		      MPLOCKED
> > +		      "cmpxchg16b %[dst];"
> > +		      " sete %[res]"
> > +		      : [dst] "=m" (dst->val[0]),
> > +			"=A" (exp->val[0]),
> > +			[res] "=r" (res)
> > +		      : "c" (src->val[1]),
> > +			"b" (src->val[0]),
> > +			"m" (dst->val[0]),
> > +			"d" (exp->val[1]),
> > +			"a" (exp->val[0])
> > +		      : "memory");
> > +
> > +	return res;
> > +}
> > +
> >  #endif /* _RTE_ATOMIC_X86_64_H_ */
> > diff --git a/lib/librte_eal/common/include/generic/rte_atomic.h
> > b/lib/librte_eal/common/include/generic/rte_atomic.h
> > index b99ba4688..8d612d566 100644
> > --- a/lib/librte_eal/common/include/generic/rte_atomic.h
> > +++ b/lib/librte_eal/common/include/generic/rte_atomic.h
> > @@ -14,6 +14,7 @@
> >
> >  #include <stdint.h>
> >  #include <rte_common.h>
> > +#include <rte_compat.h>
> >
> >  #ifdef __DOXYGEN__
> >
> > @@ -1082,4 +1083,68 @@ static inline void
> > rte_atomic64_clear(rte_atomic64_t
> > *v)
> >  }
> >  #endif
> >
> > +/*------------------------ 128 bit atomic operations
> > +------------------------
> > -*/
> > +
> > +/**
> > + * 128-bit integer structure.
> > + */
> > +typedef struct {
> > +	uint64_t val[2];
> > +} __rte_aligned(16) rte_int128_t;
> So we can't use __int128?
> 

I'll put it in a union with val[2], in case any implementations want to use it.

Thanks,
Gage

[snip]