Re: [PATCH v3 1/3] eal: add lcore poll busyness telemetry

[Jerin Jacob <jerinjacobk@gmail.com>]

On Thu, Aug 25, 2022 at 8:56 PM Kevin Laatz <kevin.laatz@intel.com> wrote:
>
> From: Anatoly Burakov <anatoly.burakov@intel.com>
>
> Currently, there is no way to measure lcore poll busyness in a passive way,
> without any modifications to the application. This patch adds a new EAL API
> that will be able to passively track core polling busyness.
>
> The poll busyness is calculated by relying on the fact that most DPDK API's
> will poll for packets. Empty polls can be counted as "idle", while
> non-empty polls can be counted as busy. To measure lcore poll busyness, we
> simply call the telemetry timestamping function with the number of polls a
> particular code section has processed, and count the number of cycles we've
> spent processing empty bursts. The more empty bursts we encounter, the less
> cycles we spend in "busy" state, and the less core poll busyness will be
> reported.
>
> In order for all of the above to work without modifications to the
> application, the library code needs to be instrumented with calls to the
> lcore telemetry busyness timestamping function. The following parts of DPDK
> are instrumented with lcore telemetry calls:
>
> - All major driver API's:
>   - ethdev
>   - cryptodev
>   - compressdev
>   - regexdev
>   - bbdev
>   - rawdev
>   - eventdev
>   - dmadev
> - Some additional libraries:
>   - ring
>   - distributor
>
> To avoid performance impact from having lcore telemetry support, a global
> variable is exported by EAL, and a call to timestamping function is wrapped
> into a macro, so that whenever telemetry is disabled, it only takes one
> additional branch and no function calls are performed. It is also possible
> to disable it at compile time by commenting out RTE_LCORE_BUSYNESS from
> build config.
>
> This patch also adds a telemetry endpoint to report lcore poll busyness, as
> well as telemetry endpoints to enable/disable lcore telemetry. A
> documentation entry has been added to the howto guides to explain the usage
> of the new telemetry endpoints and API.
>
> Signed-off-by: Kevin Laatz <kevin.laatz@intel.com>
> Signed-off-by: Conor Walsh <conor.walsh@intel.com>
> Signed-off-by: David Hunt <david.hunt@intel.com>
> Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
>
> ---
> v3:
>   * Fix missed renaming to poll busyness
>   * Fix clang compilation
>   * Fix arm compilation
>
> v2:
>   * Use rte_get_tsc_hz() to adjust the telemetry period
>   * Rename to reflect polling busyness vs general busyness
>   * Fix segfault when calling telemetry timestamp from an unregistered
>     non-EAL thread.
>   * Minor cleanup
> ---

> diff --git a/meson_options.txt b/meson_options.txt
> index 7c220ad68d..725b851f69 100644
> --- a/meson_options.txt
> +++ b/meson_options.txt
> @@ -20,6 +20,8 @@ option('enable_driver_sdk', type: 'boolean', value: false, description:
>         'Install headers to build drivers.')
>  option('enable_kmods', type: 'boolean', value: false, description:
>         'build kernel modules')
> +option('enable_lcore_poll_busyness', type: 'boolean', value: true, description:
> +       'enable collection of lcore poll busyness telemetry')

IMO, All fastpath features should be opt-in. i.e default should be false.
For the trace fastpath related changes, We have done the similar thing
even though it cost additional one cycle for disabled trace points



>  option('examples', type: 'string', value: '', description:
>         'Comma-separated list of examples to build by default')
>  option('flexran_sdk', type: 'string', value: '', description:
> --
> 2.31.1
>