Re: [dpdk-dev] [PATCH] vdpa/mlx5: fix polling threads scheduling

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 2/8/21 10:28 AM, Matan Azrad wrote:
> When the event mode is with 0 fixed delay, the polling-thread will never
> give-up CPU.
> 
> So, when multi-polling-threads are active, the context-switch between
> them will be managed by the system which may affect latency according to
> the time-out decided by the system.
> 
> In order to fix multi-devices polling thread scheduling, this patch
> forces rescheduling for each CQ poll iteration.
> 
> Move the polling thread to SCHED_RR mode with maximum priority to
> complete the fairness.
> 
> Fixes: 6956a48cabbb ("vdpa/mlx5: set polling mode default delay to zero")
> 
> Signed-off-by: Matan Azrad <matan@nvidia.com>
> ---
>  drivers/vdpa/mlx5/mlx5_vdpa_event.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/drivers/vdpa/mlx5/mlx5_vdpa_event.c b/drivers/vdpa/mlx5/mlx5_vdpa_event.c
> index 0f635ff..86adc86 100644
> --- a/drivers/vdpa/mlx5/mlx5_vdpa_event.c
> +++ b/drivers/vdpa/mlx5/mlx5_vdpa_event.c
> @@ -232,6 +232,9 @@
>  	}
>  	if (priv->timer_delay_us)
>  		usleep(priv->timer_delay_us);
> +	else
> +		/* Give-up CPU to improve polling threads scheduling. */
> +		pthread_yield();
>  }
>  
>  static void *
> @@ -500,6 +503,9 @@
>  	rte_cpuset_t cpuset;
>  	pthread_attr_t attr;
>  	char name[16];
> +	const struct sched_param sp = {
> +		.sched_priority = sched_get_priority_max(SCHED_RR),
> +	};
>  
>  	if (!priv->eventc)
>  		/* All virtqs are in poll mode. */
> @@ -520,6 +526,16 @@
>  			DRV_LOG(ERR, "Failed to set thread affinity.");
>  			return -1;
>  		}
> +		ret = pthread_attr_setschedpolicy(&attr, SCHED_RR);
> +		if (ret) {
> +			DRV_LOG(ERR, "Failed to set thread sched policy = RR.");
> +			return -1;
> +		}
> +		ret = pthread_attr_setschedparam(&attr, &sp);
> +		if (ret) {
> +			DRV_LOG(ERR, "Failed to set thread priority.");
> +			return -1;
> +		}
>  		ret = pthread_create(&priv->timer_tid, &attr,
>  				     mlx5_vdpa_poll_handle, (void *)priv);
>  		if (ret) {
> 

Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>