Re: [dpdk-dev] [PATCH v7 1/3] eal/ticketlock: ticket based to improve fairness

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Joyce,

> 
> 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>
> ---
>  MAINTAINERS                                        |   4 +
>  doc/api/doxy-api-index.md                          |   1 +
>  lib/librte_eal/common/Makefile                     |   2 +-
>  .../common/include/generic/rte_ticketlock.h        | 215 +++++++++++++++++++++
>  lib/librte_eal/common/meson.build                  |   1 +
>  5 files changed, 222 insertions(+), 1 deletion(-)
>  create mode 100644 lib/librte_eal/common/include/generic/rte_ticketlock.h
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 452b8eb..3521271 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
> +
>  ARM v7
>  M: Jan Viktorin <viktorin@rehivetech.com>
>  M: Gavin Hu <gavin.hu@arm.com>
> diff --git a/doc/api/doxy-api-index.md b/doc/api/doxy-api-index.md
> index d95ad56..aacc66b 100644
> --- a/doc/api/doxy-api-index.md
> +++ b/doc/api/doxy-api-index.md
> @@ -65,6 +65,7 @@ The public API headers are grouped by topics:
>    [atomic]             (@ref rte_atomic.h),
>    [rwlock]             (@ref rte_rwlock.h),
>    [spinlock]           (@ref rte_spinlock.h)
> +  [ticketlock]         (@ref rte_ticketlock.h)
> 
>  - **CPU arch**:
>    [branch prediction]  (@ref rte_branch_prediction.h),
> diff --git a/lib/librte_eal/common/Makefile b/lib/librte_eal/common/Makefile
> index c487201..ac3305c 100644
> --- a/lib/librte_eal/common/Makefile
> +++ b/lib/librte_eal/common/Makefile
> @@ -20,7 +20,7 @@ INC += rte_bitmap.h rte_vfio.h rte_hypervisor.h rte_test.h
>  INC += rte_reciprocal.h rte_fbarray.h rte_uuid.h
> 
>  GENERIC_INC := rte_atomic.h rte_byteorder.h rte_cycles.h rte_prefetch.h
> -GENERIC_INC += rte_spinlock.h rte_memcpy.h rte_cpuflags.h rte_rwlock.h
> +GENERIC_INC += rte_spinlock.h rte_memcpy.h rte_cpuflags.h rte_rwlock.h rte_ticketlock.h
>  GENERIC_INC += rte_vect.h rte_pause.h rte_io.h
> 
>  # defined in mk/arch/$(RTE_ARCH)/rte.vars.mk
> diff --git a/lib/librte_eal/common/include/generic/rte_ticketlock.h b/lib/librte_eal/common/include/generic/rte_ticketlock.h
> new file mode 100644
> index 0000000..c5203cb
> --- /dev/null
> +++ b/lib/librte_eal/common/include/generic/rte_ticketlock.h
> @@ -0,0 +1,215 @@
> +/* SPDX-License-Identifier: BSD-3-Clause
> + * Copyright(c) 2019 Arm Limited
> + */
> +
> +#ifndef _RTE_TICKETLOCK_H_
> +#define _RTE_TICKETLOCK_H_
> +
> +/**
> + * @file
> + *
> + * RTE ticket locks
> + *
> + * This file defines an API for ticket locks, which give each waiting
> + * thread a ticket and take the lock one by one, first come, first
> + * serviced.
> + *
> + * All locks must be initialised before use, and only initialised once.
> + *
> + */
> +
> +#ifdef __cplusplus
> +extern "C" {
> +#endif
> +
> +#include <rte_common.h>
> +#include <rte_lcore.h>
> +#include <rte_pause.h>
> +
> +/**
> + * The rte_ticketlock_t type.
> + */
> +typedef union {
> +	uint32_t tickets;
> +	struct {
> +		uint16_t current;
> +		uint16_t next;
> +	} s;
> +} rte_ticketlock_t;
> +
> +/**
> + * A static ticketlock initializer.
> + */
> +#define RTE_TICKETLOCK_INITIALIZER { 0 }
> +
> +/**
> + * Initialize the ticketlock to an unlocked state.
> + *
> + * @param tl
> + *   A pointer to the ticketlock.
> + */
> +static inline __rte_experimental void
> +rte_ticketlock_init(rte_ticketlock_t *tl)
> +{
> +	__atomic_store_n(&tl->tickets, 0, __ATOMIC_RELAXED);
> +}
> +
> +/**
> + * Take the ticketlock.
> + *
> + * @param tl
> + *   A pointer to the ticketlock.
> + */
> +static inline __rte_experimental void
> +rte_ticketlock_lock(rte_ticketlock_t *tl)
> +{
> +	uint16_t me = __atomic_fetch_add(&tl->s.next, 1, __ATOMIC_RELAXED);
> +	while (__atomic_load_n(&tl->s.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)
> +{
> +	uint16_t i = __atomic_load_n(&tl->s.current, __ATOMIC_RELAXED);
> +	__atomic_store_n(&tl->s.current, i + 1, __ATOMIC_RELEASE);
> +}
> +
> +/**
> + * Try to take the lock.
> + *
> + * @param tl
> + *   A pointer to the ticketlock.
> + * @return
> + *   1 if the lock is successfully taken; 0 otherwise.
> + */
> +static inline __rte_experimental int
> +rte_ticketlock_trylock(rte_ticketlock_t *tl)
> +{
> +	rte_ticketlock_t old, new;
> +	old.tickets = __atomic_load_n(&tl->tickets, __ATOMIC_RELAXED);
> +	new.tickets = __atomic_load_n(&tl->tickets, __ATOMIC_RELAXED);

As a nit:  from my point, there is no need for second load, we can just do:
new.tickets = old.tickets;

Apart from that:
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>


> +	new.s.next++;
> +	if (old.s.next == old.s.current) {
> +		if (__atomic_compare_exchange_n(&tl->tickets, &old.tickets,
> +		    new.tickets, 0, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
> +			return 1;
> +	}
> +
> +	return 0;
> +}
> +