Re: [PATCH] eal: add cache guard to per-lcore PRNG state

[Thomas Monjalon <thomas@monjalon.net>]

TLS is an alternative solution proposed by Stephen.
What do you think?


06/09/2023 18:25, Stephen Hemminger:
> On Mon, 4 Sep 2023 13:57:19 +0200
> Mattias Rönnblom <hofors@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.ronnblom@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 <stephen@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 <stephen@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
>