netdev - Re: [PATCH v2 net-next 3/3] netem: support delivering packets in delayed time slots

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Message-ID: <1510180826.2849.117.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Wed, 08 Nov 2017 14:40:26 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Dave Taht <dave.taht@...il.com>
Cc:     netdev@...r.kernel.org, stephen@...workplumber.org
Subject: Re: [PATCH v2 net-next 3/3] netem: support delivering packets in
 delayed time slots

On Wed, 2017-11-08 at 13:09 -0800, Dave Taht wrote:
> Slotting is a crude approximation of the behaviors of shared media such
> as cable, wifi, and LTE, which gather up a bunch of packets within a
> varying delay window and deliver them, relative to that, nearly all at
> once.

> -		qdisc_watchdog_schedule_ns(&q->watchdog, time_to_send);
> +
> +		if (q->slot.slot_next > now)
> +			qdisc_watchdog_schedule_ns(&q->watchdog,
> +						   q->slot.slot_next);
> +		else
> +			qdisc_watchdog_schedule_ns(&q->watchdog, time_to_send);


This looks weird.

You might incur an extra timer event in the following case :

now < q->slot.slot_next < time_to_send

I would rather do

qdisc_watchdog_schedule_ns(&q->watchdog,
                           max(time_to_send, q->slot.slot_next));

So that a single timer expiration is needed.