Re: [dpdk-dev] [PATCH v7] eal: add cache-line demote support

[David Marchand <david.marchand@redhat.com>]

Repeating my questions:
- would there be a point in hinting at where the "demoted" line goes?
- is this instruction available on all x86 CPUs?


See comments:

On Tue, Oct 13, 2020 at 6:47 PM Omkar Maslekar <omkar.maslekar@intel.com> wrote:
> diff --git a/app/test/test_prefetch.c b/app/test/test_prefetch.c
> index 41f219a..5c58d0c 100644
> --- a/app/test/test_prefetch.c
> +++ b/app/test/test_prefetch.c
> @@ -26,7 +26,11 @@
>         rte_prefetch1(&a);
>         rte_prefetch2(&a);
>
> +/* test for marking a line as shared to test cldemote functionality */

Non indented comment that gives no more info than the call itself.
Please remove.

> +       rte_cldemote(&a);
> +
>         return 0;
>  }
>
> +

Please remove this empty line.


>  REGISTER_TEST_COMMAND(prefetch_autotest, test_prefetch);
> diff --git a/doc/guides/rel_notes/release_20_11.rst b/doc/guides/rel_notes/release_20_11.rst
> index b7881f2..8a1ed01 100644
> --- a/doc/guides/rel_notes/release_20_11.rst
> +++ b/doc/guides/rel_notes/release_20_11.rst
> @@ -171,6 +171,13 @@ New Features
>    * Extern objects and functions can be plugged into the pipeline.
>    * Transaction-oriented table updates.
>
> +* **Added new function rte_cldemote in rte_prefetch.h.**
> +
> +  Added a hardware hint CLDEMOTE, which is similar to prefetch in reverse.

This should come at the top of the features list (but after "write
combining store" entry that got in first).

Please add a mention that it only concerns x86.


> +  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.
> +
>
>  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..28b3d48 100644
> --- a/lib/librte_eal/arm/include/rte_prefetch_32.h
> +++ b/lib/librte_eal/arm/include/rte_prefetch_32.h
> @@ -10,6 +10,7 @@
>  #endif
>
>  #include <rte_common.h>
> +#include <rte_compat.h>

Move rte_compat.h inclusion from the arch headers to the
generic/rte_prefetch.h header only.


>  #include "generic/rte_prefetch.h"
>
>  static inline void rte_prefetch0(const volatile void *p)
> @@ -33,6 +34,12 @@ static inline void rte_prefetch_non_temporal(const volatile void *p)
>         rte_prefetch0(p);
>  }
>
> +__rte_experimental
> +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..1c722eb 100644
> --- a/lib/librte_eal/arm/include/rte_prefetch_64.h
> +++ b/lib/librte_eal/arm/include/rte_prefetch_64.h
> @@ -10,6 +10,7 @@
>  #endif
>
>  #include <rte_common.h>
> +#include <rte_compat.h>
>  #include "generic/rte_prefetch.h"
>
>  static inline void rte_prefetch0(const volatile void *p)
> @@ -32,6 +33,12 @@ static inline void rte_prefetch_non_temporal(const volatile void *p)
>         asm volatile ("PRFM PLDL1STRM, [%0]" : : "r" (p));
>  }
>
> +__rte_experimental
> +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..ad9844c 100644
> --- a/lib/librte_eal/include/generic/rte_prefetch.h
> +++ b/lib/librte_eal/include/generic/rte_prefetch.h
> @@ -51,4 +51,19 @@
>   */
>  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 move the cache line to the more

guaranteed*


> + * remote cache, where it expects sharing to be efficient and to indicate that a
> + * line may be accessed by a different core in the future.
> + *
> + * @param p
> + *   Address to demote
> + */
> +__rte_experimental
> +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..b55cac4 100644
> --- a/lib/librte_eal/ppc/include/rte_prefetch.h
> +++ b/lib/librte_eal/ppc/include/rte_prefetch.h
> @@ -11,6 +11,7 @@
>  #endif
>
>  #include <rte_common.h>
> +#include <rte_compat.h>
>  #include "generic/rte_prefetch.h"
>
>  static inline void rte_prefetch0(const volatile void *p)
> @@ -34,6 +35,12 @@ static inline void rte_prefetch_non_temporal(const volatile void *p)
>         rte_prefetch0(p);
>  }
>
> +__rte_experimental
> +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..92ba05a 100644
> --- a/lib/librte_eal/x86/include/rte_prefetch.h
> +++ b/lib/librte_eal/x86/include/rte_prefetch.h
> @@ -10,6 +10,7 @@
>  #endif
>
>  #include <rte_common.h>
> +#include <rte_compat.h>
>  #include "generic/rte_prefetch.h"
>
>  static inline void rte_prefetch0(const volatile void *p)
> @@ -32,6 +33,16 @@ 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

We use

> + * versions support this instruction natively.
> + */
> +__rte_experimental
> +static inline void rte_cldemote(const volatile void *p)
> +{
> +       asm volatile(".byte 0x0f, 0x1c, 0x06" :: "S" (p));
> +}
> +
>  #ifdef __cplusplus
>  }
>  #endif
> --
> 1.8.3.1
>


-- 
David Marchand