On Mon, 4 Sep 2023 13:57:19 +0200
Mattias Rönnblom <hof...@lysator.liu.se> wrote:
On 2023-09-04 11:26, Morten Brørup wrote:
The per-lcore random state is frequently updated by their individual
lcores, so add a cache guard to prevent CPU cache thrashing.
"to prevent false sharing in case the CPU employs a next-N-lines (or
similar) hardware prefetcher"
In my world, cache trashing and cache line contention are two different
things.
Other than that,
Acked-by: Mattias Rönnblom <mattias.ronnb...@ericsson.com>
Could the per-lcore state be thread local?
Something like this:
From 3df5e28a7e5589d05e1eade62a0979e84697853d Mon Sep 17 00:00:00 2001
From: Stephen Hemminger <step...@networkplumber.org>
Date: Wed, 6 Sep 2023 09:22:42 -0700
Subject: [PATCH] random: use per lcore state
Move the random number state into thread local storage.
This has a several benefits.
- no false cache sharing from cpu prefetching
- fixes initialization of random state for non-DPDK threads
- fixes unsafe usage of random state by non-DPDK threads
The initialization of random number state is done by the
lcore (lazy initialization).
Signed-off-by: Stephen Hemminger <step...@networkplumber.org>
---
lib/eal/common/rte_random.c | 35 +++++++++++++++++------------------
1 file changed, 17 insertions(+), 18 deletions(-)
diff --git a/lib/eal/common/rte_random.c b/lib/eal/common/rte_random.c
index 53636331a27b..62f36038ac52 100644
--- a/lib/eal/common/rte_random.c
+++ b/lib/eal/common/rte_random.c
@@ -19,13 +19,14 @@ struct rte_rand_state {
uint64_t z3;
uint64_t z4;
uint64_t z5;
-} __rte_cache_aligned;
+ uint64_t seed;
+};
-/* One instance each for every lcore id-equipped thread, and one
- * additional instance to be shared by all others threads (i.e., all
- * unregistered non-EAL threads).
- */
-static struct rte_rand_state rand_states[RTE_MAX_LCORE + 1];
+/* Global random seed */
+static uint64_t rte_rand_seed;
+
+/* Per lcore random state. */
+static RTE_DEFINE_PER_LCORE(struct rte_rand_state, rte_rand_state);
static uint32_t
__rte_rand_lcg32(uint32_t *seed)
@@ -76,16 +77,14 @@ __rte_srand_lfsr258(uint64_t seed, struct rte_rand_state
*state)
state->z3 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 4096UL);
state->z4 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 131072UL);
state->z5 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 8388608UL);
+
+ state->seed = seed;
}
void
rte_srand(uint64_t seed)
{
- unsigned int lcore_id;
-
- /* add lcore_id to seed to avoid having the same sequence */
- for (lcore_id = 0; lcore_id < RTE_MAX_LCORE; lcore_id++)
- __rte_srand_lfsr258(seed + lcore_id, &rand_states[lcore_id]);
+ __atomic_store_n(&rte_rand_seed, seed, __ATOMIC_RELAXED);
}
static __rte_always_inline uint64_t
@@ -119,15 +118,15 @@ __rte_rand_lfsr258(struct rte_rand_state *state)
static __rte_always_inline
struct rte_rand_state *__rte_rand_get_state(void)
{
- unsigned int idx;
-
- idx = rte_lcore_id();
+ struct rte_rand_state *rand_state = &RTE_PER_LCORE(rte_rand_state);
+ uint64_t seed;
- /* last instance reserved for unregistered non-EAL threads */
- if (unlikely(idx == LCORE_ID_ANY))
- idx = RTE_MAX_LCORE;
+ /* did seed change */
+ seed = __atomic_load_n(&rte_rand_seed, __ATOMIC_RELAXED);
+ if (unlikely(seed != rand_state->seed))
+ __rte_srand_lfsr258(seed, rand_state);
- return &rand_states[idx];
+ return rand_state;
}
uint64_t
