[PATCH] eal: fix core_index for non-EAL registered threads

[Maxime Peim]

Threads registered via rte_thread_register() are assigned a valid
lcore_id by eal_lcore_non_eal_allocate(), but their core_index in
lcore_config is left at -1. This value was set during rte_eal_cpu_init()
for lcores with ROLE_OFF (undetected CPUs) and is never updated when the
lcore is later allocated to a non-EAL thread.

As a result, rte_lcore_index() returns -1 for registered non-EAL
threads. Libraries that use rte_lcore_index() to select per-lcore
caches fall back to a shared global path when it returns -1, causing
severe contention under concurrent access from multiple registered
threads.

A concrete example is the mlx5 indexed memory pool (mlx5_ipool), which
uses rte_lcore_index() in mlx5_ipool_malloc_cache() to select a per-core
cache slot. When core_index is -1, all registered threads are funneled
into a single shared slot protected by a spinlock. In testing with VPP
(which registers worker threads via rte_thread_register()), this caused
async flow rule insertion throughput to drop from ~6.4M rules/sec to
~1.2M rules/sec with 4 workers -- a 5x regression attributable entirely
to spinlock contention in the ipool allocator.

Fix by setting core_index to the next sequential index (cfg->lcore_count)
in eal_lcore_non_eal_allocate() before incrementing the count. Also reset
core_index back to -1 on the error rollback path and in
eal_lcore_non_eal_release() for correctness.

Fixes: 5c307ba2a5b1 ("eal: register non-EAL threads as lcores")
Signed-off-by: Maxime Peim <maxime.peim at gmail.com>
---
 lib/eal/common/eal_common_lcore.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/eal/common/eal_common_lcore.c b/lib/eal/common/eal_common_lcore.c
index 39411f9370..ae085d73e4 100644
--- a/lib/eal/common/eal_common_lcore.c
+++ b/lib/eal/common/eal_common_lcore.c
@@ -378,6 +378,7 @@ eal_lcore_non_eal_allocate(void)
 	for (lcore_id = 0; lcore_id < RTE_MAX_LCORE; lcore_id++) {
 		if (cfg->lcore_role[lcore_id] != ROLE_OFF)
 			continue;
+		lcore_config[lcore_id].core_index = cfg->lcore_count;
 		cfg->lcore_role[lcore_id] = ROLE_NON_EAL;
 		cfg->lcore_count++;
 		break;
@@ -399,6 +400,7 @@ eal_lcore_non_eal_allocate(void)
 		}
 		EAL_LOG(DEBUG, "Initialization refused for lcore %u.",
 			lcore_id);
+		lcore_config[lcore_id].core_index = -1;
 		cfg->lcore_role[lcore_id] = ROLE_OFF;
 		cfg->lcore_count--;
 		lcore_id = RTE_MAX_LCORE;
@@ -420,6 +422,7 @@ eal_lcore_non_eal_release(unsigned int lcore_id)
 		goto out;
 	TAILQ_FOREACH(callback, &lcore_callbacks, next)
 		callback_uninit(callback, lcore_id);
+	lcore_config[lcore_id].core_index = -1;
 	cfg->lcore_role[lcore_id] = ROLE_OFF;
 	cfg->lcore_count--;
 out:
-- 
2.43.0