[ovs-dev] [v12 13/16] dpdk: Cache result of CPU ISA checks.

Stokes, Ian ian.stokes at intel.com
Wed Jun 9 15:23:05 UTC 2021


> As a small optimization, this patch caches the result of a CPU ISA
> check from DPDK. Particularly in the case of running the DPCLS
> autovalidator (which repeatedly probes subtables) this reduces
> the amount of CPU ISA lookups from the DPDK level.
> 
> By caching them at the OVS/dpdk.c level, the ISA checks remain
> runtime for the CPU where they are executed, but subsequent checks
> for the same ISA feature become much cheaper.
> 
> Signed-off-by: Harry van Haaren <harry.van.haaren at intel.com>
> Co-authored-by: Cian Ferriter <cian.ferriter at intel.com>
> Signed-off-by: Cian Ferriter <cian.ferriter at intel.com>
> 

Thanks for the patch Cian/Harry,

to my mind this does not have a dependency on the previous AVX512 patches? So could be merged separately regardless?

A few comments inline below also.
> ---
> 
> v8: Add NEWS entry.
> ---
>  NEWS       |  1 +
>  lib/dpdk.c | 28 ++++++++++++++++++++++++----
>  2 files changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/NEWS b/NEWS
> index 98943c404..c71273ddd 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -21,6 +21,7 @@ Post-v2.15.0
>     - DPDK:
>       * OVS validated with DPDK 20.11.1. It is recommended to use this version
>         until further releases.
> +     * Cache results for CPU ISA checks, reduces overhead on repeated
> lookups.
Not sure if a news Item is required for this, it seems to be a small optimization over the existing implementation and it's not really exposed to the user.

Out of interest do you have metrics regarding the overhead of the previous approach and the improvement this approach gives?


Regards
Ian
> 
> 
>  v2.15.0 - 15 Feb 2021
> diff --git a/lib/dpdk.c b/lib/dpdk.c
> index 319540394..c883a4b8b 100644
> --- a/lib/dpdk.c
> +++ b/lib/dpdk.c
> @@ -614,13 +614,33 @@ print_dpdk_version(void)
>      puts(rte_version());
>  }
> 
> +/* Avoid calling rte_cpu_get_flag_enabled() excessively, by caching the
> + * result of the call for each CPU flag in a static variable. To avoid
> + * allocating large numbers of static variables, use a uint8 as a bitfield.
> + * Note the macro must only return if the ISA check is done and available.
> + */
> +#define ISA_CHECK_DONE_BIT (1 << 0)
> +#define ISA_AVAILABLE_BIT  (1 << 1)
> +
>  #define CHECK_CPU_FEATURE(feature, name_str, RTE_CPUFLAG)               \
>      do {                                                                \
>          if (strncmp(feature, name_str, strlen(name_str)) == 0) {        \
> -            int has_isa = rte_cpu_get_flag_enabled(RTE_CPUFLAG);        \
> -            VLOG_DBG("CPU flag %s, available %s\n", name_str,           \
> -                      has_isa ? "yes" : "no");                          \
> -            return true;                                                \
> +            static uint8_t isa_check_##RTE_CPUFLAG;                     \
> +            int check = isa_check_##RTE_CPUFLAG & ISA_CHECK_DONE_BIT;   \
> +            if (OVS_UNLIKELY(!check)) {                                 \
> +                int has_isa = rte_cpu_get_flag_enabled(RTE_CPUFLAG);    \
> +                VLOG_DBG("CPU flag %s, available %s\n",                 \
> +                         name_str, has_isa ? "yes" : "no");             \
> +                isa_check_##RTE_CPUFLAG = ISA_CHECK_DONE_BIT;           \
> +                if (has_isa) {                                          \
> +                    isa_check_##RTE_CPUFLAG |= ISA_AVAILABLE_BIT;       \
> +                }                                                       \
> +            }                                                           \
> +            if (isa_check_##RTE_CPUFLAG & ISA_AVAILABLE_BIT) {          \
> +                return true;                                            \
> +            } else {                                                    \
> +                return false;                                           \
> +            }                                                           \
>          }                                                               \
>      } while (0)
> 
> --
> 2.31.1
> 
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