Re: [dpdk-dev] [PATCH v4] eal: add cache-line demote support

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Sep 21, 2020 at 06:59:27PM -0700, Omkar Maslekar wrote:
> rte_cldemote is similar to a prefetch hint - in reverse. cldemote(addr)
> enables software to hint to hardware that line is likely to be shared.
> Useful in core-to-core communications where cache-line is likely to be
> shared. ARM and PPC implementation is provided with NOP and can be added
> if any equivalent instructions could be used for implementation on those
> architectures.
> 
> Signed-off-by: Omkar Maslekar <omkar.maslekar@intel.com>
>
Few minor suggestions below. With those fixed, feel free to add my ack to
future versions of this patch.

Acked-by: Bruce Richardson <bruce.richardson@intel.com>
 
> ---
> v4: updated bold text for title and fixed margin in release notes
> *
> v3: fixed warning regarding whitespace
> *
> v2: documentation updated
> ---
> ---
>  doc/guides/rel_notes/release_20_11.rst        |  6 ++++++
>  lib/librte_eal/arm/include/rte_prefetch_32.h  |  5 +++++
>  lib/librte_eal/arm/include/rte_prefetch_64.h  |  5 +++++
>  lib/librte_eal/include/generic/rte_prefetch.h | 13 +++++++++++++
>  lib/librte_eal/ppc/include/rte_prefetch.h     |  5 +++++
>  lib/librte_eal/x86/include/rte_prefetch.h     |  9 +++++++++
>  6 files changed, 43 insertions(+)
> 
> diff --git a/doc/guides/rel_notes/release_20_11.rst b/doc/guides/rel_notes/release_20_11.rst
> index df227a1..b844b96 100644
> --- a/doc/guides/rel_notes/release_20_11.rst
> +++ b/doc/guides/rel_notes/release_20_11.rst
> @@ -55,6 +55,12 @@ New Features
>       Also, make sure to start the actual text at the margin.
>       =======================================================
>  
> +* **Added new function rte_cldemote in rte_prefetch.h.**
> +
> +  Added a hardware hint CLDEMOTE, which is similar to prefetch in reverse.
> +  CLDEMOTE moves the cache line to the more remote cache, where it expects
> +  sharing to be efficient. Moving the cache line to a level more distant from
> +  the processor helps to accelerate core-to-core communication.
>  

I think you need two blank lines between sections here, not just one.

>  Removed Items
>  -------------
> diff --git a/lib/librte_eal/arm/include/rte_prefetch_32.h b/lib/librte_eal/arm/include/rte_prefetch_32.h
> index e53420a..ad91edd 100644
> --- a/lib/librte_eal/arm/include/rte_prefetch_32.h
> +++ b/lib/librte_eal/arm/include/rte_prefetch_32.h
> @@ -33,6 +33,11 @@ static inline void rte_prefetch_non_temporal(const volatile void *p)
>  	rte_prefetch0(p);
>  }
>  
> +static inline void rte_cldemote(const volatile void *p)
> +{
> +	RTE_SET_USED(p);
> +}
> +
>  #ifdef __cplusplus
>  }
>  #endif
> diff --git a/lib/librte_eal/arm/include/rte_prefetch_64.h b/lib/librte_eal/arm/include/rte_prefetch_64.h
> index fc2b391..35d278a 100644
> --- a/lib/librte_eal/arm/include/rte_prefetch_64.h
> +++ b/lib/librte_eal/arm/include/rte_prefetch_64.h
> @@ -32,6 +32,11 @@ static inline void rte_prefetch_non_temporal(const volatile void *p)
>  	asm volatile ("PRFM PLDL1STRM, [%0]" : : "r" (p));
>  }
>  
> +static inline void rte_cldemote(const volatile void *p)
> +{
> +	RTE_SET_USED(p);
> +}
> +
>  #ifdef __cplusplus
>  }
>  #endif
> diff --git a/lib/librte_eal/include/generic/rte_prefetch.h b/lib/librte_eal/include/generic/rte_prefetch.h
> index 6e47bdf..8742412 100644
> --- a/lib/librte_eal/include/generic/rte_prefetch.h
> +++ b/lib/librte_eal/include/generic/rte_prefetch.h
> @@ -51,4 +51,17 @@
>   */
>  static inline void rte_prefetch_non_temporal(const volatile void *p);
>  
> +/**
> + * Demote a cache line to a more distant level of cache from the processor.
> + *
> + * CLDEMOTE hints to hardware to move (demote) a cache line from the closest to
> + * the processor to a level more distant from the processor. It is a hint and
> + * not guarantee. rte_cldemote is intended to speed up things at the producer,
> + * in the producer-consumer case.
> + *

Two thoughts here:
1. Is it not more the consumer who benefits more since they are the ones
receiving the demoted value, while the producer pays a higher cost since
they have to demote the value on send?
2. Rather than talking about producer consumer case specifically, I think
it would be good to replace the last sentence with what you have in the
cover letter about it being for sharing, and to indicate that a line may be
accessed by a different core in the future.

> + * @param p
> + *   Address to demote
> + */
> +static inline void rte_cldemote(const volatile void *p);
> +
>  #endif /* _RTE_PREFETCH_H_ */
> diff --git a/lib/librte_eal/ppc/include/rte_prefetch.h b/lib/librte_eal/ppc/include/rte_prefetch.h
> index 9ba07c8..3fe9655 100644
> --- a/lib/librte_eal/ppc/include/rte_prefetch.h
> +++ b/lib/librte_eal/ppc/include/rte_prefetch.h
> @@ -34,6 +34,11 @@ static inline void rte_prefetch_non_temporal(const volatile void *p)
>  	rte_prefetch0(p);
>  }
>  
> +static inline void rte_cldemote(const volatile void *p)
> +{
> +	RTE_SET_USED(p);
> +}
> +
>  #ifdef __cplusplus
>  }
>  #endif
> diff --git a/lib/librte_eal/x86/include/rte_prefetch.h b/lib/librte_eal/x86/include/rte_prefetch.h
> index 384c6b3..029d06e 100644
> --- a/lib/librte_eal/x86/include/rte_prefetch.h
> +++ b/lib/librte_eal/x86/include/rte_prefetch.h
> @@ -32,6 +32,15 @@ static inline void rte_prefetch_non_temporal(const volatile void *p)
>  	asm volatile ("prefetchnta %[p]" : : [p] "m" (*(const volatile char *)p));
>  }
>  
> +/*
> + * we're using raw byte codes for now as only the newest compiler
> + * versions support this instruction natively.
> + */
> +static inline void rte_cldemote(const volatile void *p)
> +{
> +	asm volatile(".byte 0x0f, 0x1c, 0x06" :: "S" (p));
> +}
> +
>  #ifdef __cplusplus
>  }
>  #endif
> -- 
> 1.8.3.1
>