Re: [dpdk-dev] [PATCH v5 1/2] eal/ticketlock: ticket based to improve fairness

[Jerin Jacob Kollanukkaran <jerinj@marvell.com>]

On Mon, 2019-03-11 at 13:52 +0800, Joyce Kong wrote:
> The spinlock implementation is unfair, some threads may take locks
> aggressively while leaving the other threads starving for long time.
> 
> This patch introduces ticketlock which gives each waiting thread a
> ticket and they can take the lock one by one. First come, first
> serviced.
> This avoids starvation for too long time and is more predictable.
> 
> Suggested-by: Jerin Jacob <jerinj@marvell.com>
> Signed-off-by: Joyce kong <joyce.kong@arm.com>
> Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>
> Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> ---
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 097cfb4..12a091f 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -210,6 +210,10 @@ M: Cristian Dumitrescu <
> cristian.dumitrescu@intel.com>
>  F: lib/librte_eal/common/include/rte_bitmap.h
>  F: app/test/test_bitmap.c
>  
> +Ticketlock
> +M: Joyce Kong <joyce.kong@arm.com>
> +F: lib/librte_eal/common/include/generic/rte_ticketlock.h


Add F: app/test/test_ticketlock.c in the next patch



> 
> +#include <rte_lcore.h>
> +#include <rte_common.h>
> +#include <rte_pause.h>

Sort the header in alphabetical order.


> +
> +/**
> + * The rte_ticketlock_t type.
> + */
> +typedef struct {
> +	uint16_t current;
> +	uint16_t next;
> +} rte_ticketlock_t;
> +
> 
> +
> +/**
> + * Take the ticketlock.
> + *
> + * @param tl
> + *   A pointer to the ticketlock.
> + */
> +static inline __rte_experimental void
> +rte_ticketlock_lock(rte_ticketlock_t *tl)
> +{
> +	unsigned int me = __atomic_fetch_add(&tl->next, 1, 

If current, next is uint16_t why "me" as unsigned int.


> __ATOMIC_RELAXED);
> +	while (__atomic_load_n(&tl->current, __ATOMIC_ACQUIRE) != me)
> +		rte_pause();
> +}
> +
> +/**
> + * Release the ticketlock.
> + *
> + * @param tl
> + *   A pointer to the ticketlock.
> + */
> +static inline __rte_experimental void
> +rte_ticketlock_unlock(rte_ticketlock_t *tl)
> +{
> +	unsigned int i = __atomic_load_n(&tl->current,
> __ATOMIC_RELAXED);
> +	i++;

You can save this line by making
__atomic_store_n(&tl->current, i + 1, __ATOMIC_RELEASE);


The code looks good. Please check the above comments and earlier
reported compilation issue with clang 7.1