Re: [dpdk-dev] [PATCH 1/4] test/stack: avoid trivial memory allocations

["Eads, Gage" <gage.eads@intel.com>]

Hi Steven,

> -----Original Message-----
> From: Steven Lariau <steven.lariau@arm.com>
> Sent: Wednesday, August 5, 2020 10:57 AM
> To: Eads, Gage <gage.eads@intel.com>; Olivier Matz
> <olivier.matz@6wind.com>
> Cc: dev@dpdk.org; honnappa.nagarahalli@arm.com;
> dharmik.thakkar@arm.com; nd@arm.com; Steven Lariau
> <steven.lariau@arm.com>
> Subject: [PATCH 1/4] test/stack: avoid trivial memory allocations
> 
> Replace the arguments array by one argument.
> All objects in the args array have the same values, so there is no need
> to use an array, only one struct is enough.
> The args object is a lot smaller, and the allocation can be replaced
> with a stack variable.
> 
> The allocation of obj_table isn't needed either, because MAX_BULK is
> small. The allocation can instead be replaced with a static array.
> 
> Signed-off-by: Steven Lariau <steven.lariau@arm.com>
> Reviewed-by: Dharmik Thakkar <dharmik.thakkar@arm.com>
> Reviewed-by: Phil Yang <phil.yang@arm.com>
> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> ---
>  app/test/test_stack.c | 39 ++++++---------------------------------
>  1 file changed, 6 insertions(+), 33 deletions(-)
> 
> diff --git a/app/test/test_stack.c b/app/test/test_stack.c
> index c8dac1f55..5a7273a7d 100644
> --- a/app/test/test_stack.c
> +++ b/app/test/test_stack.c
> @@ -280,16 +280,9 @@ static int
>  stack_thread_push_pop(void *args)
>  {
>  	struct test_args *t = args;
> -	void **obj_table;
> +	void *obj_table[MAX_BULK];
>  	int i;
> 
> -	obj_table = rte_calloc(NULL, STACK_SIZE, sizeof(void *), 0);
> -	if (obj_table == NULL) {
> -		printf("[%s():%u] failed to calloc %zu bytes\n",
> -		       __func__, __LINE__, STACK_SIZE * sizeof(void *));
> -		return -1;
> -	}
> -
>  	for (i = 0; i < NUM_ITERS_PER_THREAD; i++) {
>  		unsigned int success, num;
> 
> @@ -310,28 +303,25 @@ stack_thread_push_pop(void *args)
>  		if (rte_stack_push(t->s, obj_table, num) != num) {
>  			printf("[%s():%u] Failed to push %u pointers\n",
>  			       __func__, __LINE__, num);
> -			rte_free(obj_table);
>  			return -1;
>  		}
> 
>  		if (rte_stack_pop(t->s, obj_table, num) != num) {
>  			printf("[%s():%u] Failed to pop %u pointers\n",
>  			       __func__, __LINE__, num);
> -			rte_free(obj_table);
>  			return -1;
>  		}
> 
>  		rte_atomic64_sub(t->sz, num);
>  	}
> 
> -	rte_free(obj_table);
>  	return 0;
>  }

Agreed, the dynamic allocation is unnecessary.

> 
>  static int
>  test_stack_multithreaded(uint32_t flags)
>  {
> -	struct test_args *args;
> +	struct test_args args;
>  	unsigned int lcore_id;
>  	struct rte_stack *s;
>  	rte_atomic64_t size;
> @@ -344,45 +334,28 @@ test_stack_multithreaded(uint32_t flags)
>  	printf("[%s():%u] Running with %u lcores\n",
>  	       __func__, __LINE__, rte_lcore_count());
> 
> -	args = rte_malloc(NULL, sizeof(struct test_args) * RTE_MAX_LCORE,
> 0);
> -	if (args == NULL) {
> -		printf("[%s():%u] failed to malloc %zu bytes\n",
> -		       __func__, __LINE__,
> -		       sizeof(struct test_args) * RTE_MAX_LCORE);
> -		return -1;
> -	}
> -
>  	s = rte_stack_create("test", STACK_SIZE, rte_socket_id(), flags);
>  	if (s == NULL) {
>  		printf("[%s():%u] Failed to create a stack\n",
>  		       __func__, __LINE__);
> -		rte_free(args);
>  		return -1;
>  	}
> 
>  	rte_atomic64_init(&size);
> +	args.s = s;
> +	args.sz = &size;
> 
>  	RTE_LCORE_FOREACH_SLAVE(lcore_id) {
> -		args[lcore_id].s = s;
> -		args[lcore_id].sz = &size;
> -
>  		if (rte_eal_remote_launch(stack_thread_push_pop,
> -					  &args[lcore_id], lcore_id))
> +					  &args, lcore_id))
>  			rte_panic("Failed to launch lcore %d\n", lcore_id);
>  	}


In general we shouldn't pass a stack variable to other threads. Though your
code here looks fine, I'd rather err on the safe side in case this is ever used
as a template/basis for some other code...particularly since there's no
performance/correctness/etc. penalty to using dynamically allocated memory.

To support patch 2/4, you can instead convert the rte_malloc to allocate a
single shared test_args structure. Or perhaps move patch 4 earlier in the series,
and simply pass the stack pointer instead.

Thanks,
Gage