Re: [PATCH] ofnet: Remove 200 ms timeout in get_card_packet to reduce input latency

[Michael Chang]

Hi,

A gentle ping. With many patches posted these days, I hope this one is
not overlooked.

A quick note: This fixes a one second latency for each text input in the
grub interactive shell/editor when the image is net booted, as reported
on mkvterm on powerhmc, caused quite a few complaints from our internal
lab and QA.

Thanks,
Michael

On Mon, May 06, 2024 at 10:34:22AM GMT, Michael Chang wrote:
> When grub image is netbooted on ppc64le, the keyboard input exhibits
> significant latency, reports even say that characters are processed
> about once per second. This issue makes interactively trying to debug a
> ppc64le config very difficult.
> 
> It seems that the latency is largely caused by a 200 ms timeout in the
> idle event loop, during which the network card interface is consistently
> polled for incoming packets. Often, no packets arrive during this
> period, so the timeout nearly always expires, which blocks the response
> to key inputs.
> 
> Furthermore, this 200 ms timeout might not need to be enforced at this
> basic layer, considering that grub performs synchronous reads and its
> timeout management is actually handled by higher layers, not directly in
> the card instance. Additionally, the idle polling, which reacts to
> unsolicited packets like ICMP and SLAAC, would be fine at a less
> frequent polling interval, rather than needing a timeout for receiving a
> response.
> 
> For these reasons, we believe the timeout in get_card_packet should be
> effectively removed. According to test results, the delay has
> disappeared, and it is now much easier to use interactively.
> 
> Signed-Off-by: Michael Chang <mchang@suse.com>
> Tested-by: Tony Jones <tonyj@suse.com>
> ---
>  grub-core/net/drivers/ieee1275/ofnet.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/grub-core/net/drivers/ieee1275/ofnet.c 
> b/grub-core/net/drivers/ieee1275/ofnet.c
> index 78f03df8e..3bf48b3f0 100644
> --- a/grub-core/net/drivers/ieee1275/ofnet.c
> +++ b/grub-core/net/drivers/ieee1275/ofnet.c
> @@ -82,15 +82,11 @@ get_card_packet (struct grub_net_card *dev)
>    grub_ssize_t actual;
>    int rc;
>    struct grub_ofnetcard_data *data = dev->data;
> -  grub_uint64_t start_time;
>    struct grub_net_buff *nb;
>  
> -  start_time = grub_get_time_ms ();
> -  do
> -    rc = grub_ieee1275_read (data->handle, dev->rcvbuf, dev->rcvbufsize, 
> &actual);
> -  while ((actual <= 0 || rc < 0) && (grub_get_time_ms () - start_time < 
> 200));
> +  rc = grub_ieee1275_read (data->handle, dev->rcvbuf, dev->rcvbufsize, 
> &actual);
>  
> -  if (actual <= 0)
> +  if (actual <= 0 || rc < 0)
>      return NULL;
>  
>    nb = grub_netbuff_alloc (actual + 2);
> -- 
> 2.44.0
>