[v3,1/2] service: add API to retrieve service core active

  This commit adds a new experimental API which allows the user
to retrieve the active state of an lcore. Knowing when the service
lcore is completed its polling loop can be useful to applications
to avoid race conditions when e.g. finalizing statistics.

The service thread itself now has a variable to indicate if its
thread is active. When zero the service thread has completed its
service, and has returned from the service_runner_func() function.

Suggested-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>
Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>

---

v3:
- Change service lcore stores to SEQ_CST (Honnappa, David)
- Change control thread load to ACQ (Honnappa, David)
- Comment reasons for SEQ_CST/ACQ (Honnappa, David)
- Add comments to Doxygen for _stop() and _lcore_active() (Honnappa, David)
- Add Phil's review tag from ML
---
 lib/librte_eal/common/rte_service.c  | 23 ++++++++++++++++++++++-
 lib/librte_eal/include/rte_service.h | 20 +++++++++++++++++++-
 lib/librte_eal/rte_eal_version.map   |  1 +
 3 files changed, 42 insertions(+), 2 deletions(-)
  

<snip>

> Subject: [PATCH v3 1/2] service: add API to retrieve service core active
> 
> This commit adds a new experimental API which allows the user to retrieve
> the active state of an lcore. Knowing when the service lcore is completed its
> polling loop can be useful to applications to avoid race conditions when e.g.
> finalizing statistics.
> 
> The service thread itself now has a variable to indicate if its thread is active.
> When zero the service thread has completed its service, and has returned
> from the service_runner_func() function.
> 
> Suggested-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>
> Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
> Reviewed-by: Phil Yang <phil.yang@arm.com>

Over all looks good, few nits inline.
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

> 
> ---
> 
> v3:
> - Change service lcore stores to SEQ_CST (Honnappa, David)
> - Change control thread load to ACQ (Honnappa, David)
> - Comment reasons for SEQ_CST/ACQ (Honnappa, David)
> - Add comments to Doxygen for _stop() and _lcore_active() (Honnappa, David)
> - Add Phil's review tag from ML
> ---
>  lib/librte_eal/common/rte_service.c  | 23 ++++++++++++++++++++++-
> lib/librte_eal/include/rte_service.h | 20 +++++++++++++++++++-
>  lib/librte_eal/rte_eal_version.map   |  1 +
>  3 files changed, 42 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/librte_eal/common/rte_service.c
> b/lib/librte_eal/common/rte_service.c
> index 6a0e0ff65..b0cf7c4ab 100644
> --- a/lib/librte_eal/common/rte_service.c
> +++ b/lib/librte_eal/common/rte_service.c
> @@ -65,6 +65,7 @@ struct core_state {
>  	/* map of services IDs are run on this core */
>  	uint64_t service_mask;
>  	uint8_t runstate; /* running or stopped */
> +	uint8_t thread_active; /* indicates when thread is in service_run() */
>  	uint8_t is_service_core; /* set if core is currently a service core */
>  	uint8_t service_active_on_lcore[RTE_SERVICE_NUM_MAX];
>  	uint64_t loops;
> @@ -422,7 +423,7 @@ rte_service_may_be_active(uint32_t id)
>  		return -EINVAL;
> 
>  	for (i = 0; i < lcore_count; i++) {
> -		if (lcore_states[ids[i]].service_active_on_lcore[id])
> +	if (lcore_states[ids[i]].service_active_on_lcore[id])
nit, I think the <tab> is required here.

>  			return 1;
>  	}
> 
> @@ -457,6 +458,8 @@ service_runner_func(void *arg)
>  	const int lcore = rte_lcore_id();
>  	struct core_state *cs = &lcore_states[lcore];
> 
> +	__atomic_store_n(&cs->thread_active, 1, __ATOMIC_SEQ_CST);
> +
>  	/* runstate act as the guard variable. Use load-acquire
>  	 * memory order here to synchronize with store-release
>  	 * in runstate update functions.
> @@ -475,9 +478,27 @@ service_runner_func(void *arg)
>  		cs->loops++;
>  	}
> 
> +	/* Use SEQ CST memory ordering to avoid any re-ordering around
> +	 * this store, ensuring that once this store is visible, the service
> +	 * lcore thread really is done in service cores code.
> +	 */
> +	__atomic_store_n(&cs->thread_active, 0, __ATOMIC_SEQ_CST);
>  	return 0;
>  }
> 
> +int32_t
> +rte_service_lcore_active(uint32_t lcore) {
> +	if (lcore >= RTE_MAX_LCORE || !lcore_states[lcore].is_service_core)
> +		return -EINVAL;
> +
> +	/* Load thread_active using ACQUIRE to avoid instructions dependent
> on
> +	 * the result being re-ordered before this load completes.
> +	 */
> +	return __atomic_load_n(&lcore_states[lcore].thread_active,
> +			       __ATOMIC_ACQUIRE);
> +}
> +
>  int32_t
>  rte_service_lcore_count(void)
>  {
> diff --git a/lib/librte_eal/include/rte_service.h
> b/lib/librte_eal/include/rte_service.h
> index e2d0a6dd3..84245020d 100644
> --- a/lib/librte_eal/include/rte_service.h
> +++ b/lib/librte_eal/include/rte_service.h
> @@ -249,7 +249,9 @@ int32_t rte_service_lcore_start(uint32_t lcore_id);
>   * Stop a service core.
>   *
>   * Stopping a core makes the core become idle, but remains  assigned as a
> - * service core.
> + * service core. Note that the service lcore thread may not have
> + returned from
> + * the service it is running when this API returns. To check if the
> + service
> + * lcore is still active, the *rte_service_lcore_active* API is added.
nit
Instead of "To check if the service lcore is still active, the *rte_service_lcore_active* API is added.", may be "*rte_service_lcore_active* API can be used to check if the service lcore is still active"?

>   *
>   * @retval 0 Success
>   * @retval -EINVAL Invalid *lcore_id* provided @@ -261,6 +263,22 @@
> int32_t rte_service_lcore_start(uint32_t lcore_id);
>   */
>  int32_t rte_service_lcore_stop(uint32_t lcore_id);
> 
> +/**
> + * Reports if a service lcore is currently running.
nit                                                     ^^^^^^^^^^^^^^^ may be "currently running mapped services"? (matching with the documentation for existing APIs)

> + *
> + * This function returns if the core has finished service cores code,
nit                                                                     ^^^^^^^^^^^^^^^^^^^^^^ may be "completed running assigned services"?

> +and has
> + * returned to EAL control. If *rte_service_lcore_stop* has been called
> +but
> + * the lcore has not returned to EAL yet, it might be required to wait
> +and call
> + * this function again. The amount of time to wait before the core
> +returns
> + * depends on the duration of the services being run.
> + *
> + * @retval 0 Service thread is not active, and has been returned to EAL.
nit,                                    ^^^^^ lcore?
> + * @retval 1 Service thread is in the service core polling loop.
nit, may be "Service lcore is running assigned services."? (matching the terms used in existing APIs)

> + * @retval -EINVAL Invalid *lcore_id* provided.
> + */
> +__rte_experimental
> +int32_t rte_service_lcore_active(uint32_t lcore_id);
> +
>  /**
>   * Adds lcore to the list of service cores.
>   *
> diff --git a/lib/librte_eal/rte_eal_version.map
> b/lib/librte_eal/rte_eal_version.map
> index bf0c17c23..d53d5d5b9 100644
> --- a/lib/librte_eal/rte_eal_version.map
> +++ b/lib/librte_eal/rte_eal_version.map
> @@ -401,6 +401,7 @@ EXPERIMENTAL {
>  	rte_lcore_dump;
>  	rte_lcore_iterate;
>  	rte_mp_disable;
> +	rte_service_lcore_active;
>  	rte_thread_register;
>  	rte_thread_unregister;
>  };
> --
> 2.17.1