RE: [PATCH v3 1/2] eal: add thread yield functions

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Thomas Monjalon [mailto:thomas@monjalon.net]
> Sent: Wednesday, 25 October 2023 18.31
> 
> When running real-time threads, we may need to force scheduling
> kernel threads or other real-time threads.
> New functions are added to address these cases.
> 
> The yield functions should not have any interest for normal threads.
> Note: other purposes may be addressed with rte_pause() or rte_delay_*().
> 
> Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
> ---
>  lib/eal/include/rte_thread.h | 22 ++++++++++++++++++++++
>  lib/eal/unix/rte_thread.c    | 16 ++++++++++++++++
>  lib/eal/version.map          |  4 ++++
>  lib/eal/windows/rte_thread.c | 15 +++++++++++++++
>  4 files changed, 57 insertions(+)
> 
> diff --git a/lib/eal/include/rte_thread.h b/lib/eal/include/rte_thread.h
> index 8da9d4d3fb..139cafac96 100644
> --- a/lib/eal/include/rte_thread.h
> +++ b/lib/eal/include/rte_thread.h
> @@ -183,6 +183,28 @@ int rte_thread_join(rte_thread_t thread_id,
> uint32_t *value_ptr);
>   */
>  int rte_thread_detach(rte_thread_t thread_id);
> 
> +/**
> + * Allow another thread to run on the same CPU core.
> + *
> + * Lower priority threads may not be scheduled.
> + *
> + * Especially useful in real-time thread priority
> + * to schedule other real-time threads.
> + * @see RTE_THREAD_PRIORITY_REALTIME_CRITICAL
> + */
> +__rte_experimental
> +void rte_thread_yield(void);
> +
> +/**
> + * Unblock a CPU core running busy in a real-time thread.
> + *
> + * Especially useful in real-time thread priority
> + * to avoid a busy loop blocking vital threads on a core.
> + * @see RTE_THREAD_PRIORITY_REALTIME_CRITICAL
> + */
> +__rte_experimental
> +void rte_thread_yield_realtime(void);
> +

If an application really needs to use real-time priority, the behavior of any DPDK yield functions must be documented in much more detail than this - especially in regard to expected latency.

[...]

> diff --git a/lib/eal/windows/rte_thread.c b/lib/eal/windows/rte_thread.c
> index acf648456c..1e031eca40 100644
> --- a/lib/eal/windows/rte_thread.c
> +++ b/lib/eal/windows/rte_thread.c
> @@ -304,6 +304,21 @@ rte_thread_detach(rte_thread_t thread_id)
>  	return 0;
>  }
> 
> +void
> +rte_thread_yield(void)
> +{
> +	Sleep(0);
> +}
> +
> +void
> +rte_thread_yield_realtime(void)
> +{
> +	/* Real-time threads are not causing problems on Windows.
> +	 * A normal yield should be fine.

Back in the days, the Windows API had a Yield() function; make sure your comment can't be misunderstood as referring to that.

> +	 */
> +	Sleep(0);
> +}