Subject: Re: [PATCH net v2 1/2] net: sched: tbf: fix calculation of max_size

From: Jesper Dangaard Brouer <[email-masked]>
Date: Mon, 18 Nov 2013 13:32:46 +0100

On Mon, 18 Nov 2013 16:39:23 +0800
Yang Yingliang <[email-masked]> wrote:

> commit b757c9336d63f94c6b57532(tbf: improved accuracy at high rates)
> introduce a regression.
> 
> With the follow command:
> tc qdisc add dev eth1 root handle 1: tbf latency 50ms burst 10KB rate 30gbit mtu 64k
> 
> Without this patch, the max_size value is 10751(bytes).
> But, in fact, the real max_size value should be smaller than 7440(bytes).
> Or a packet whose length is bigger than 7440 will cause network congestion.
> Because the packet is so big that can't get enough tokens. Even all the tokens
> in the buffer is given to the packet.
> 
> With this patch, the max_size value is 7440(bytes).
> The packets whose length is bigger than 7440(bytes) will be dropped or reshape
> in tbf_enqueue().

I acknowledge that TBF seems to have some dependencies to the
userspace constructed rate table (which we do NOT use anymore in the
kernel).  And that these should be fixed.

But I'm not sure that your patch is the best solution... and the patch
also contains some issues, see inlined comments.

The main annoying problem is *again* how the rate table system got
removed, in the kernel, but nobody fixed userspace.

So, the main problem is that qopt->buffer (send from userspace/tc) is
in a "time-format" (user input "burst" in bytes).  Which used-to, make
sense because the rate table used the same "time-format".

Now you are reversing this calculation of "q->buffer" (token burst)
back into bytes, so we can choose "max_size" (to avoid a problem in
tbf_dequeue()).

I don't like this converting back-and-forth, I'm worried about
rounding errors.

The easiest "hack" solution would be:

 for (n = 0; n < 65535; n++)
     if (psched_l2t_ns(&qopt->rate, n) > q->buffer)
         break;
 max_size = n;

Unfortunately we have to keep backward compat with iproute2/tc, but
IMHO it would be a lot easier, if we could fix userspace, and remove
all the length-to-time calculations, as they should now be the
responsibility of the kernel.  Well, wishful thinking...

> Signed-off-by: Yang Yingliang <[email-masked]>
> ---
>  include/net/sch_generic.h | 46 ++++++++++++++++++++++++++++++++
>  net/sched/sch_tbf.c       | 67 ++++++++++++++++++++++++++---------------------
>  2 files changed, 83 insertions(+), 30 deletions(-)
> 
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index d0a6321..8da64f3 100644
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -701,6 +701,52 @@ static inline u64 psched_l2t_ns(const struct psched_ratecfg *r,
>  	return ((u64)len * r->mult) >> r->shift;
>  }
>  
> +/* Time to Length, convert time in ns to length in bytes
> + * to determinate how many bytes can be sent in given time.
> + */
> +static inline u64 psched_ns_t2l(const struct psched_ratecfg *r,
> +				u64 time_in_ns)
> +{
> +	u64 len = time_in_ns;
> +	u8 shift = r->shift;
> +	bool is_div = false;
> +
> +	/* The formula is :
> +	 * len = (time_in_ns << shift) / mult
> +	 * when time_in_ns does shift, it would overflow.
> +	 * If overflow happens first time, do division.
> +	 * Then do shift. If it happens again,
> +	 * set lenth to ~0ULL.
> +	 */
> +	while (shift) {
> +		if (len & (1ULL << 63)) {
> +			if (!is_div) {
> +				len = div64_u64(len, r->mult);
> +				is_div = true;
> +			} else {
> +				/* overflow happens */
> +				len = ~0ULL;
> +				is_div = true;
> +				break;
> +			}
> +		}
> +		len <<= 1;
> +		shift--;
> +	}
> +	if (!is_div)
> +		len = div64_u64(len, r->mult);
> +
> +	if (unlikely(r->linklayer == TC_LINKLAYER_ATM))
> +		len = (len / 53) * 48;
> +
> +	if (len > r->overhead)
> +		len -= r->overhead;
> +	else
> +		len = 0;
> +
> +	return len;
> +}
> +

Are we 100% sure, that the conversion is accurate between
psched_l2t_ns() and psched_ns_t2l for all possible rates.

E.g. why is it that r->shift have to be recalculate (orig created in
psched_ratecfg_precompute()).

>  void psched_ratecfg_precompute(struct psched_ratecfg *r,
>  			       const struct tc_ratespec *conf,
>  			       u64 rate64);
> diff --git a/net/sched/sch_tbf.c b/net/sched/sch_tbf.c
> index 68f9859..eb9ce7b 100644
> --- a/net/sched/sch_tbf.c
> +++ b/net/sched/sch_tbf.c
> @@ -279,7 +279,7 @@ static int tbf_change(struct Qdisc *sch, struct nlattr *opt)
>  	struct qdisc_rate_table *rtab = NULL;
>  	struct qdisc_rate_table *ptab = NULL;
>  	struct Qdisc *child = NULL;
> -	int max_size, n;
> +	u32 max_size = 0;
>  	u64 rate64 = 0, prate64 = 0;
>  
>  	err = nla_parse_nested(tb, TCA_TBF_MAX, opt, tbf_policy);
> @@ -291,33 +291,20 @@ static int tbf_change(struct Qdisc *sch, struct nlattr *opt)
>  		goto done;
>  
>  	qopt = nla_data(tb[TCA_TBF_PARMS]);
> -	rtab = qdisc_get_rtab(&qopt->rate, tb[TCA_TBF_RTAB]);
> -	if (rtab == NULL)
> -		goto done;
> -
> -	if (qopt->peakrate.rate) {
> -		if (qopt->peakrate.rate > qopt->rate.rate)
> -			ptab = qdisc_get_rtab(&qopt->peakrate, tb[TCA_TBF_PTAB]);
> -		if (ptab == NULL)
> -			goto done;
> +	if (qopt->rate.linklayer == TC_LINKLAYER_UNAWARE) {
> +		rtab = qdisc_get_rtab(&qopt->rate, tb[TCA_TBF_RTAB]);
> +		if (rtab) {
> +			qdisc_put_rtab(rtab);
> +			rtab = NULL;
> +		}

This is correct code construct, for backward compatible reading of the
rate table.

But, notice how your free this at once, which means you also should
cleanup the exit/done: section.

>  	}
> -
> -	for (n = 0; n < 256; n++)
> -		if (rtab->data[n] > qopt->buffer)
> -			break;
> -	max_size = (n << qopt->rate.cell_log) - 1;

This is here we could do the quick-and-dirty solution:

 for (n = 0; n < 65535; n++)
     if (psched_l2t_ns(&qopt->rate, n) > q->buffer)
         break;
 max_size = n;

> -	if (ptab) {
> -		int size;
> -
> -		for (n = 0; n < 256; n++)
> -			if (ptab->data[n] > qopt->mtu)
> -				break;
> -		size = (n << qopt->peakrate.cell_log) - 1;
> -		if (size < max_size)
> -			max_size = size;
> +	if (qopt->peakrate.linklayer == TC_LINKLAYER_UNAWARE) {
> +		ptab = qdisc_get_rtab(&qopt->peakrate, tb[TCA_TBF_PTAB]);
> +		if (ptab) {
> +			qdisc_put_rtab(ptab);
> +			ptab = NULL;
> +		}
>  	}
> -	if (max_size < 0)
> -		goto done;
>  
>  	if (q->qdisc != &noop_qdisc) {
>  		err = fifo_set_limit(q->qdisc, qopt->limit);
> @@ -339,25 +326,45 @@ 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;
> +	max_size = min_t(u64, psched_ns_t2l(&q->rate, q->buffer), ~0);
> +	max_size = min_t(u32, max_size, (256 << qopt->rate.cell_log) - 1);

The rate system and the rate.cell_log is not really used anymore, so
its a bit strange to use it.  Perhaps it's even a bug to base a
calculation on this.

> +
> +	if (qopt->peakrate.rate) {
> +		u64 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);
> +		size = psched_ns_t2l(&q->peak, q->mtu);
> +		max_size = min_t(u64, max_size, size);
> +		max_size = min_t(u32,
> +				 max_size,
> +				 (256 << qopt->peakrate.cell_log) - 1);
>  		q->peak_present = true;
>  	} else {
>  		q->peak_present = false;
>  	}
>  
> +	if (!max_size)
> +		goto unlock_done;
> +	q->max_size = max_size;
> +
>  	sch_tree_unlock(sch);
>  	err = 0;
> +
> +	if (0) {

I really dislike this construct.  I'm afraid what a dumb compile would
optimized this to.

> +unlock_done:
> +		sch_tree_unlock(sch);
> +		err = -EINVAL;
> +	}
>  done:
>  	if (rtab)
>  		qdisc_put_rtab(rtab);

This could be cleaned up, as we already have released the rtab's.

-- 
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