Re: [dpdk-dev] [PATCH v2 2/2] mempool/nb_stack: add non-blocking stack mempool

[Andrew Rybchenko <arybchenko@solarflare.com>]

On 1/16/19 1:32 AM, Gage Eads wrote:
> This commit adds support for non-blocking (linked list based) stack mempool
> handler. The stack uses a 128-bit compare-and-swap instruction, and thus is
> limited to x86_64. The 128-bit CAS atomically updates the stack top pointer
> and a modification counter, which protects against the ABA problem.
>
> In mempool_perf_autotest the lock-based stack outperforms the non-blocking
> handler*, however:
> - For applications with preemptible pthreads, a lock-based stack's
>    worst-case performance (i.e. one thread being preempted while
>    holding the spinlock) is much worse than the non-blocking stack's.
> - Using per-thread mempool caches will largely mitigate the performance
>    difference.
>
> *Test setup: x86_64 build with default config, dual-socket Xeon E5-2699 v4,
> running on isolcpus cores with a tickless scheduler. The lock-based stack's
> rate_persec was 1x-3.5x the non-blocking stack's.
>
> Signed-off-by: Gage Eads <gage.eads@intel.com>
> ---

Few minor nits below. Other than that
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>

Don't forget about release notes when 19.05 release cycle starts.

[snip]

> diff --git a/drivers/mempool/meson.build b/drivers/mempool/meson.build
> index 4527d9806..01ee30fee 100644
> --- a/drivers/mempool/meson.build
> +++ b/drivers/mempool/meson.build
> @@ -2,6 +2,11 @@
>   # Copyright(c) 2017 Intel Corporation
>   
>   drivers = ['bucket', 'dpaa', 'dpaa2', 'octeontx', 'ring', 'stack']
> +
> +if dpdk_conf.has('RTE_ARCH_X86_64')
> +	drivers += 'nb_stack'
> +endif
> +

I think it would be better to concentrate the logic inside 
nb_stack/meson.build.
There is a 'build' variable which may be set to false disable the build.
You can find an example in drivers/net/sfc/meson.build.

[snip]

> +static __rte_always_inline void
> +nb_lifo_push(struct nb_lifo *lifo,
> +	     struct nb_lifo_elem *first,
> +	     struct nb_lifo_elem *last,
> +	     unsigned int num)
> +{
> +	while (1) {
> +		struct nb_lifo_head old_head, new_head;
> +
> +		old_head = lifo->head;
> +
> +		/* Swing the top pointer to the first element in the list and
> +		 * make the last element point to the old top.
> +		 */
> +		new_head.top = first;
> +		new_head.cnt = old_head.cnt + 1;
> +
> +		last->next = old_head.top;
> +
> +		if (rte_atomic128_cmpset((volatile uint64_t *) &lifo->head,

Unnecessary space after type cast above.

[snip]

> +		new_head.top = tmp;
> +		new_head.cnt = old_head.cnt + 1;
> +
> +		if (rte_atomic128_cmpset((volatile uint64_t *) &lifo->head,

Unnecessary space after type cast above.

[snip]