Subject: Re: [PATCH net v3 1/2] net: sched: tbf: fix calculation of max_size

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 20 Nov 2013 11:04:48 +0100

On Wed, 20 Nov 2013 10:14:43 +0800
Yang Yingliang <[email-masked]> wrote:
> On 2013/11/19 17:38, Jesper Dangaard Brouer wrote:
> > On Tue, 19 Nov 2013 15:25:38 +0800
> > Yang Yingliang <[email-masked]> wrote:
> > 
[...]
> > 
> >> diff --git a/net/sched/sch_tbf.c b/net/sched/sch_tbf.c
> >> index 68f9859..c194129 100644
> >> --- a/net/sched/sch_tbf.c
> >> +++ b/net/sched/sch_tbf.c
> > [...]
> >> @@ -339,30 +326,46 @@ static int tbf_change(struct Qdisc *sch, struct nlattr *opt)
> > [...]
> >> +	for (n = 0; n < 65536; n++)
> >> +		if (psched_l2t_ns(&q->rate, n) > q->buffer)
> >> +			break;
> >> +	max_size = min_t(u32, n, (256ULL << qopt->rate.cell_log) - 1);
> > 
> > I'm a little uncertain about, if using the 65536 constant is okay, or
> > considered "bad style".
> 
> I'll use a MACRO to instead of this constant in v4.

I'm not saying you have to use a macro for this. I'm fine with using
65536 here, some other developers might have stronger opinions on this?

> > I'm still a little confused/uncertain why we need the "qopt->rate.cell_log".
> > 
> 
> Because we need max_size be smaller than mtu(user input in bytes).
> E.g. if user inputs like this "...  burst 100KB rate 100mbit mtu 4095",
> without this patch, max_size is 4095.
> But with this patch, if don't use cell_log, max_size is 102400.
> I think it's not correct, so I used cell_log here.
 
Hmmmm... so you are using cell_log to approximate the MTU size, because
struct tc_ratespec, does not contain the a mtu parameter.  You do
realize this is only an approximation. (This also smells like a bad
transition away from the userspace rate tables)

What about the p->mtu parameter from struct tc_tbf_qopt.  I know it is
in a "time" format... but hey, you are already using this parameter
requoting your patch:

On Tue, 19 Nov 2013 15:25:38 +0800
Yang Yingliang <[email-masked]> wrote:

> diff --git a/net/sched/sch_tbf.c b/net/sched/sch_tbf.c
> index 68f9859..c194129 100644
> --- a/net/sched/sch_tbf.c
> +++ b/net/sched/sch_tbf.c
[... cut ...]
> @@ -339,30 +326,46 @@ static int tbf_change(struct Qdisc *sch, struct nlattr *opt)
>  	}
>  	q->limit = qopt->limit;
>  	q->mtu = PSCHED_TICKS2NS(qopt->mtu);
> -	q->max_size = max_size;
>  	q->buffer = PSCHED_TICKS2NS(qopt->buffer);
>  	q->tokens = q->buffer;
>  	q->ptokens = q->mtu;
>  
>  	if (tb[TCA_TBF_RATE64])
>  		rate64 = nla_get_u64(tb[TCA_TBF_RATE64]);
> -	psched_ratecfg_precompute(&q->rate, &rtab->rate, rate64);
> -	if (ptab) {
> +	psched_ratecfg_precompute(&q->rate, &qopt->rate, rate64);
> +	if (!q->rate.rate_bytes_ps)
> +		goto unlock_done;
> +	for (n = 0; n < 65536; n++)
> +		if (psched_l2t_ns(&q->rate, n) > q->buffer)
> +			break;
> +	max_size = min_t(u32, n, (256ULL << qopt->rate.cell_log) - 1);
> +
> +	if (qopt->peakrate.rate) {
> +		int size = 0;
>  		if (tb[TCA_TBF_PRATE64])
>  			prate64 = nla_get_u64(tb[TCA_TBF_PRATE64]);
> -		psched_ratecfg_precompute(&q->peak, &ptab->rate, prate64);
> +		psched_ratecfg_precompute(&q->peak, &qopt->peakrate, prate64);
> +		for (n = 0; n < 65536; n++)
> +			if (psched_l2t_ns(&q->peak, n) > q->mtu)

You are using q->mtu here, BUT why are you only doing this, when there
is a qopt->peakrate.rate, set???  (I might have missed something)

> +				break;
> +		size = min_t(u32, n, (256ULL << qopt->rate.cell_log) - 1);
> +		max_size = min_t(u32, max_size, size);
>  		q->peak_present = true;
>  	} else {
>  		q->peak_present = false;
>  	}

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer

For easy reference
------------------
(include/uapi/linux/pkt_sched.h)
struct tc_ratespec {
	unsigned char	cell_log;
	__u8		linklayer; /* lower 4 bits */
	unsigned short	overhead;
	short		cell_align;
	unsigned short	mpu;
	__u32		rate;
};

struct tc_tbf_qopt {
	struct tc_ratespec rate;
	struct tc_ratespec peakrate;
	__u32		limit;
	__u32		buffer;
	__u32		mtu;
};