Re: [PATCH] usb: f_ncm: only first packet of aggregate needs to start timer

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Jun 08, 2021 at 01:54:38AM -0700, Maciej Żenczykowski wrote:
> From: Maciej Żenczykowski <maze@google.com>
> 
> The reasoning for this change is that if we already had
> a packet pending, then we also already had a pending timer,
> and as such there is no need to reschedule it.
> 
> This also prevents packets getting delayed 60 ms worst case
> under a tiny packet every 290us transmit load, by keeping the
> timeout always relative to the first queued up packet.
> (300us delay * 16KB max aggregation / 80 byte packet =~ 60 ms)
> 
> As such the first packet is now at most delayed by 300us.
> 
> Under low transmit load, this will simply result in us sending
> a shorter aggregate, as originally intended.
> 
> This patch has the benefit of greatly reducing (by ~10 factor
> with 1500 byte frames aggregated into 16 kiB) the number of
> (potentially pretty costly) updates to the hrtimer.
> 
> Cc: Brooke Basile <brookebasile@gmail.com>
> Cc: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
> Cc: Felipe Balbi <balbi@kernel.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Lorenzo Colitti <lorenzo@google.com>
> Signed-off-by: Maciej Żenczykowski <maze@google.com>
> ---
>  drivers/usb/gadget/function/f_ncm.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/f_ncm.c b/drivers/usb/gadget/function/f_ncm.c
> index 0d23c6c11a13..855127249f24 100644
> --- a/drivers/usb/gadget/function/f_ncm.c
> +++ b/drivers/usb/gadget/function/f_ncm.c
> @@ -1101,11 +1101,11 @@ static struct sk_buff *ncm_wrap_ntb(struct gether *port,
>  			ncm->ndp_dgram_count = 1;
>  
>  			/* Note: we skip opts->next_ndp_index */
> -		}
>  
> -		/* Delay the timer. */
> -		hrtimer_start(&ncm->task_timer, TX_TIMEOUT_NSECS,
> -			      HRTIMER_MODE_REL_SOFT);
> +			/* Start the timer. */
> +			hrtimer_start(&ncm->task_timer, TX_TIMEOUT_NSECS,
> +				      HRTIMER_MODE_REL_SOFT);
> +		}
>  
>  		/* Add the datagram position entries */
>  		ntb_ndp = skb_put_zero(ncm->skb_tx_ndp, dgram_idx_len);

Nice, hopefully this helps out a lot on the systems where re-arming
timers are slow.

greg k-h