Re: [PATCH] Add upper bound to receiver ring poll to reduce DPC latency

[Paul Durrant <xadimgnik@xxxxxxxxx>]

On 07/02/2023 14:05, Rachel.Yan@xxxxxxxxxx wrote:
> From: Rachel Yan <Rachel.Yan@xxxxxxxxxx>
>
> Adds an upper bound to the ring poll iteration with optimal value found through 
> experimentation to avoid going round the ring an infinite (or very large) 
> number of times. This has been tested to show improvements in DPC latencies and 
> file transfer speeds.
Do you have numbers you can share? The reason for moving to threaded 
DPCs was to avoid having to play games like this.
  Paul


> Signed-off-by: Rachel Yan <rachel.yan@xxxxxxxxxx>
>
> ---
>   src/xenvif/receiver.c | 15 ++++++++++++++-
>   1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/src/xenvif/receiver.c b/src/xenvif/receiver.c
> index 2145133..d469de4 100644
> --- a/src/xenvif/receiver.c
> +++ b/src/xenvif/receiver.c
> @@ -2013,11 +2013,15 @@ ReceiverRingPoll(
>       PXENVIF_RECEIVER            Receiver;
>       PXENVIF_FRONTEND            Frontend;
>       ULONG                       Count;
> +    ULONG                       MaxCount;
> +    BOOLEAN                     NeedQueueDpc;
>   
>       Receiver = Ring->Receiver;
>       Frontend = Receiver->Frontend;
>   
>       Count = 0;
> +    MaxCount = 10 * XENVIF_RECEIVER_RING_SIZE;
> +    NeedQueueDpc = FALSE;
>   
>       if (!Ring->Enabled || Ring->Paused)
>           goto done;
> @@ -2068,6 +2072,15 @@ ReceiverRingPoll(
>               PXENVIF_RECEIVER_FRAGMENT   Fragment;
>               PMDL                        Mdl;
>   
> +            // avoid going through the ring an infinite  (or very large) amount of times
> +            // if the netback producer happens to fill in just enough packets 
> to cause us
> +            // to go around the ring multiple times. This should reduce Dpc 
> latencies.
> +
> +            if (Count >= MaxCount) {
> +                NeedQueueDpc = TRUE;
> +                break;
> +            }
> +
>               rsp = RING_GET_RESPONSE(&Ring->Front, rsp_cons);
>   
>               // netback is required to complete requests in order and place
> @@ -2247,7 +2260,7 @@ ReceiverRingPoll(
>       if (!__ReceiverRingIsStopped(Ring))
>           ReceiverRingFill(Ring);
>   
> -    if (Ring->PacketQueue != NULL &&
> +    if ((NeedQueueDpc || Ring->PacketQueue != NULL) &&
>           KeInsertQueueDpc(&Ring->QueueDpc, NULL, NULL))
>           Ring->QueueDpcs++;
>