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

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

On Wed, Jun 09, 2021 at 10:27:48AM +0200, Greg Kroah-Hartman wrote:
> 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.

And that's what the changelog said, I need more coffee...