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SubjectRe: [PATCH net-next v4] sched: Add dualpi2 qdisc
Date
> +static s64 __scale_delta(s64 diff)
> +{
> + do_div(diff, (1 << (ALPHA_BETA_GRANULARITY + 1)) - 1);
> + return diff;
> +}
[...]
> + delta = __scale_delta(((s64)qdelay - q->pi2.target) * q->pi2.alpha);
> + delta += __scale_delta(((s64)qdelay - qdelay_old) * q->pi2.beta);

I just noticed that ensuring 64b divide compatibility across platforms
using do_div() introduced this bug, as do_div() works with unsigned operands.

This will be fixed in a later v5 with the following patch:

---
net/sched/sch_dualpi2.c | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/net/sched/sch_dualpi2.c b/net/sched/sch_dualpi2.c
index a6452aa82018..c6c851499d35 100644
--- a/net/sched/sch_dualpi2.c
+++ b/net/sched/sch_dualpi2.c
@@ -385,7 +385,7 @@ static struct sk_buff *dualpi2_qdisc_dequeue(struct Qdisc *sch)
return skb;
}

-static s64 __scale_delta(s64 diff)
+static s64 __scale_delta(u64 diff)
{
do_div(diff, (1 << (ALPHA_BETA_GRANULARITY + 1)) - 1);
return diff;
@@ -406,16 +406,18 @@ static u32 calculate_probability(struct Qdisc *sch)
/* Alpha and beta take at most 32b, i.e, the delay difference would
* overflow for queueing delay differences > ~4.2sec.
*/
- delta = __scale_delta(((s64)qdelay - q->pi2.target) * q->pi2.alpha);
- delta += __scale_delta(((s64)qdelay - qdelay_old) * q->pi2.beta);
- new_prob = delta + q->pi2.prob;
+ delta = ((s64)qdelay - q->pi2.target) * q->pi2.alpha;
+ delta += ((s64)qdelay - qdelay_old) * q->pi2.beta;
/* Prevent overflow */
if (delta > 0) {
+ new_prob = __scale_delta(delta) + q->pi2.prob;
if (new_prob < q->pi2.prob)
new_prob = MAX_PROB;
+ } else {
+ new_prob = q->pi2.prob - __scale_delta(delta * -1);
/* Prevent underflow */
- } else if (new_prob > q->pi2.prob) {
- new_prob = 0;
+ if (new_prob > q->pi2.prob)
+ new_prob = 0;
}
/* If we do not drop on overload, ensure we cap the L4S probability to
* 100% to keep window fairness when overflowing.
--
Sorry for this.


Best,
Olivier
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