[v3,2/6] service: identify service running on another core correctly

Message ID 20200502000245.11071-3-honnappa.nagarahalli@arm.com
State Superseded
Headers show
Series
  • use c11 atomics for service core lib
Related show

Commit Message

Honnappa Nagarahalli May 2, 2020, 12:02 a.m.
The logic to identify if the MT unsafe service is running on another
core can return -EBUSY spuriously. In such cases, running the service
becomes costlier than using atomic operations. Assume that the
application passes the right parameters and reduces the number of
instructions for all cases.

Cc: stable@dpdk.org
Fixes: 8d39d3e237c2 ("service: fix race in service on app lcore function")

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Reviewed-by: Phil Yang <phil.yang@arm.com>

---
 lib/librte_eal/common/rte_service.c | 26 ++++++++------------------
 1 file changed, 8 insertions(+), 18 deletions(-)

-- 
2.17.1

Comments

Van Haaren, Harry May 5, 2020, 2:48 p.m. | #1
> -----Original Message-----

> From: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

> Sent: Saturday, May 2, 2020 1:03 AM

> To: dev@dpdk.org; phil.yang@arm.com; Van Haaren, Harry

> <harry.van.haaren@intel.com>

> Cc: thomas@monjalon.net; david.marchand@redhat.com; Ananyev, Konstantin

> <konstantin.ananyev@intel.com>; jerinj@marvell.com;

> hemant.agrawal@nxp.com; Eads, Gage <gage.eads@intel.com>; Richardson,

> Bruce <bruce.richardson@intel.com>; honnappa.nagarahalli@arm.com;

> nd@arm.com; stable@dpdk.org

> Subject: [PATCH v3 2/6] service: identify service running on another core

> correctly

>

> The logic to identify if the MT unsafe service is running on another

> core can return -EBUSY spuriously. In such cases, running the service

> becomes costlier than using atomic operations. Assume that the

> application passes the right parameters and reduces the number of

> instructions for all cases.

> 

> Cc: stable@dpdk.org

> Fixes: 8d39d3e237c2 ("service: fix race in service on app lcore function")


Add "fix" to the title, suggestion:
service: fix identification of service running on other lcore

> Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

> Reviewed-by: Phil Yang <phil.yang@arm.com>


I believe there may be some optimizations we can apply after this patchset
as the "num_mapped_cores" variable is no longer used in a significant way 
for the atomic selection, however lets leave that optimization outside
of 20.05 scope.

With title (see above) & comment (see below) updated.
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>


> ---

<snip some diff>
> @@ -412,24 +412,14 @@ rte_service_run_iter_on_app_lcore(uint32_t id,

> uint32_t serialize_mt_unsafe)

> 

>  	SERVICE_VALID_GET_OR_ERR_RET(id, s, -EINVAL);

> 

> -	/* Atomically add this core to the mapped cores first, then examine if

> -	 * we can run the service. This avoids a race condition between

> -	 * checking the value, and atomically adding to the mapped count.

> +	/* Increment num_mapped_cores to indicate that the service

> +	 * is running on a core.

>  	 */

> -	if (serialize_mt_unsafe)

> -		rte_atomic32_inc(&s->num_mapped_cores);

> +	rte_atomic32_inc(&s->num_mapped_cores);


The comment for the added lines here are a little confusing to me,
the "num_mapped_cores" does not indicate that the service "is running on a core",
it indicates the number of mapped lcores to that service. Suggestion below?

/* Increment num_mapped_cores to reflect that this core is
 * now mapped capable of running the service.
 */

> -	if (service_mt_safe(s) == 0 &&

> -			rte_atomic32_read(&s->num_mapped_cores) > 1) {

> -		if (serialize_mt_unsafe)

> -			rte_atomic32_dec(&s->num_mapped_cores);

> -		return -EBUSY;

> -	}

> -

> -	int ret = service_run(id, cs, UINT64_MAX, s);

> +	int ret = service_run(id, cs, UINT64_MAX, s, serialize_mt_unsafe);


<snip rest of diff>

Patch

diff --git a/lib/librte_eal/common/rte_service.c b/lib/librte_eal/common/rte_service.c
index b8c465eb9..c89472b83 100644
--- a/lib/librte_eal/common/rte_service.c
+++ b/lib/librte_eal/common/rte_service.c
@@ -360,7 +360,7 @@  rte_service_runner_do_callback(struct rte_service_spec_impl *s,
 /* Expects the service 's' is valid. */
 static int32_t
 service_run(uint32_t i, struct core_state *cs, uint64_t service_mask,
-	    struct rte_service_spec_impl *s)
+	    struct rte_service_spec_impl *s, uint32_t serialize_mt_unsafe)
 {
 	if (!s)
 		return -EINVAL;
@@ -374,7 +374,7 @@  service_run(uint32_t i, struct core_state *cs, uint64_t service_mask,
 
 	cs->service_active_on_lcore[i] = 1;
 
-	if (service_mt_safe(s) == 0) {
+	if ((service_mt_safe(s) == 0) && (serialize_mt_unsafe == 1)) {
 		if (!rte_atomic32_cmpset((uint32_t *)&s->execute_lock, 0, 1))
 			return -EBUSY;
 
@@ -412,24 +412,14 @@  rte_service_run_iter_on_app_lcore(uint32_t id, uint32_t serialize_mt_unsafe)
 
 	SERVICE_VALID_GET_OR_ERR_RET(id, s, -EINVAL);
 
-	/* Atomically add this core to the mapped cores first, then examine if
-	 * we can run the service. This avoids a race condition between
-	 * checking the value, and atomically adding to the mapped count.
+	/* Increment num_mapped_cores to indicate that the service
+	 * is running on a core.
 	 */
-	if (serialize_mt_unsafe)
-		rte_atomic32_inc(&s->num_mapped_cores);
+	rte_atomic32_inc(&s->num_mapped_cores);
 
-	if (service_mt_safe(s) == 0 &&
-			rte_atomic32_read(&s->num_mapped_cores) > 1) {
-		if (serialize_mt_unsafe)
-			rte_atomic32_dec(&s->num_mapped_cores);
-		return -EBUSY;
-	}
-
-	int ret = service_run(id, cs, UINT64_MAX, s);
+	int ret = service_run(id, cs, UINT64_MAX, s, serialize_mt_unsafe);
 
-	if (serialize_mt_unsafe)
-		rte_atomic32_dec(&s->num_mapped_cores);
+	rte_atomic32_dec(&s->num_mapped_cores);
 
 	return ret;
 }
@@ -449,7 +439,7 @@  rte_service_runner_func(void *arg)
 			if (!service_valid(i))
 				continue;
 			/* return value ignored as no change to code flow */
-			service_run(i, cs, service_mask, service_get(i));
+			service_run(i, cs, service_mask, service_get(i), 1);
 		}
 
 		cs->loops++;