Re: [dpdk-dev] [PATCH 1/2] mbuf: fix performance/cache resource issue with 128-byte cache line targets

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Jerin,

> -----Original Message-----
> From: Jerin Jacob [mailto:jerin.jacob@caviumnetworks.com]
> Sent: Sunday, December 06, 2015 3:59 PM
> To: dev@dpdk.org
> Cc: thomas.monjalon@6wind.com; Richardson, Bruce; olivier.matz@6wind.com; Dumitrescu, Cristian; Ananyev, Konstantin; Jerin
> Jacob
> Subject: [dpdk-dev] [PATCH 1/2] mbuf: fix performance/cache resource issue with 128-byte cache line targets
> 
> No need to split mbuf structure to two cache lines for 128-byte cache line
> size targets as it can fit on a single 128-byte cache line.
> 
> Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> ---
>  app/test/test_mbuf.c                                          | 4 ++++
>  lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h | 4 ++++
>  lib/librte_mbuf/rte_mbuf.h                                    | 2 ++
>  3 files changed, 10 insertions(+)
> 
> diff --git a/app/test/test_mbuf.c b/app/test/test_mbuf.c
> index b32bef6..5e21075 100644
> --- a/app/test/test_mbuf.c
> +++ b/app/test/test_mbuf.c
> @@ -930,7 +930,11 @@ test_failing_mbuf_sanity_check(void)
>  static int
>  test_mbuf(void)
>  {
> +#if RTE_CACHE_LINE_SIZE == 64
>  	RTE_BUILD_BUG_ON(sizeof(struct rte_mbuf) != RTE_CACHE_LINE_SIZE * 2);
> +#elif RTE_CACHE_LINE_SIZE == 128
> +	RTE_BUILD_BUG_ON(sizeof(struct rte_mbuf) != RTE_CACHE_LINE_SIZE);
> +#endif
> 
>  	/* create pktmbuf pool if it does not exist */
>  	if (pktmbuf_pool == NULL) {
> diff --git a/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h b/lib/librte_eal/linuxapp/eal/include/exec-
> env/rte_kni_common.h
> index bd1cc09..e724af7 100644
> --- a/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h
> +++ b/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h
> @@ -121,8 +121,12 @@ struct rte_kni_mbuf {
>  	uint32_t pkt_len;       /**< Total pkt len: sum of all segment data_len. */
>  	uint16_t data_len;      /**< Amount of data in segment buffer. */
> 
> +#if RTE_CACHE_LINE_SIZE == 64
>  	/* fields on second cache line */
>  	char pad3[8] __attribute__((__aligned__(RTE_CACHE_LINE_SIZE)));
> +#elif RTE_CACHE_LINE_SIZE == 128
> +	char pad3[24];
> +#endif
>  	void *pool;
>  	void *next;
>  };
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index f234ac9..0bf55e0 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -813,8 +813,10 @@ struct rte_mbuf {
> 
>  	uint16_t vlan_tci_outer;  /**< Outer VLAN Tag Control Identifier (CPU order) */
> 
> +#if RTE_CACHE_LINE_SIZE == 64
>  	/* second cache line - fields only used in slow path or on TX */
>  	MARKER cacheline1 __rte_cache_aligned;
> +#endif

I suppose you'll need to keep same space reserved for first 64B even on systems with 128B cache-line.
Otherwise we can endup with different mbuf format for systems with 128B cache-line.
Another thing - now we have __rte_cache_aligned all over the places, and I don't know is to double 
sizes of all these structures is a good idea. 
Again,  #if RTE_CACHE_LINE_SIZE == 64 ... all over the places looks a bit clumsy.
Wonder can we have __rte_cache_aligned/ RTE_CACHE_LINE_SIZE architecture specific,
but introduce RTE_CACHE_MIN_LINE_SIZE(==64)/ __rte_cache_min_aligned and used it for mbuf
(and might be other places).
Konstantin