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pie: remove pie_vars->accu_prob_overflows

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The variable pie_vars->accu_prob is used as an accumulator for
probability values. Since probabilty values are scaled using the
MAX_PROB macro denoting (2^64 - 1), pie_vars->accu_prob is
likely to overflow as it is of type u64.

The variable pie_vars->accu_prob_overflows counts the number of
times the variable pie_vars->accu_prob overflows.

The MAX_PROB macro needs to be equal to at least (2^39 - 1) in
order to do precise calculations without any underflow. Thus
MAX_PROB can be reduced to (2^56 - 1) without affecting the
precision in calculations drastically. Doing so will eliminate
the need for the variable pie_vars->accu_prob_overflows as the
variable pie_vars->accu_prob will never overflow.

Removing the variable pie_vars->accu_prob_overflows also reduces
the size of the structure pie_vars to exactly 64 bytes.

Signed-off-by: Mohit P. Tahiliani <tahiliani@nitk.edu.in>
Signed-off-by: Gautam Ramakrishnan <gautamramk@gmail.com>
Signed-off-by: Leslie Monis <lesliemonis@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Leslie Monis
2020-03-05 00:26:01 +05:30
committed by David S. Miller
parent 220d4ac74e
commit 105e808c1d
3 changed files with 7 additions and 20 deletions
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21
net/sched/sch_pie.c
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@@ -62,27 +62,19 @@ bool pie_drop_early(struct Qdisc *sch, struct pie_params *params,
else
local_prob = vars->prob;
if (local_prob == 0) {
if (local_prob == 0)
vars->accu_prob = 0;
vars->accu_prob_overflows = 0;
}
else
vars->accu_prob += local_prob;
if (local_prob > MAX_PROB - vars->accu_prob)
vars->accu_prob_overflows++;
vars->accu_prob += local_prob;
if (vars->accu_prob_overflows == 0 &&
vars->accu_prob < (MAX_PROB / 100) * 85)
if (vars->accu_prob < (MAX_PROB / 100) * 85)
return false;
if (vars->accu_prob_overflows == 8 &&
vars->accu_prob >= MAX_PROB / 2)
if (vars->accu_prob >= (MAX_PROB / 2) * 17)
return true;
prandom_bytes(&rnd, 8);
if (rnd < local_prob) {
if ((rnd >> BITS_PER_BYTE) < local_prob) {
vars->accu_prob = 0;
vars->accu_prob_overflows = 0;
return true;
}
@@ -129,7 +121,6 @@ static int pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
out:
q->stats.dropped++;
q->vars.accu_prob = 0;
q->vars.accu_prob_overflows = 0;
return qdisc_drop(skb, sch, to_free);
}