linux/include/net/red.h
Eric Dumazet ea6a5d3b97 sch_red: fix red_calc_qavg_from_idle_time
Since commit a4a710c4a7 (pkt_sched: Change PSCHED_SHIFT from 10 to
6) it seems RED/GRED are broken.

red_calc_qavg_from_idle_time() computes a delay in us units, but this
delay is now 16 times bigger than real delay, so the final qavg result
smaller than expected.

Use standard kernel time services since there is no need to obfuscate
them.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
2011-11-30 23:27:22 -05:00

319 lines
7.5 KiB
C

#ifndef __NET_SCHED_RED_H
#define __NET_SCHED_RED_H
#include <linux/types.h>
#include <net/pkt_sched.h>
#include <net/inet_ecn.h>
#include <net/dsfield.h>
/* Random Early Detection (RED) algorithm.
=======================================
Source: Sally Floyd and Van Jacobson, "Random Early Detection Gateways
for Congestion Avoidance", 1993, IEEE/ACM Transactions on Networking.
This file codes a "divisionless" version of RED algorithm
as written down in Fig.17 of the paper.
Short description.
------------------
When a new packet arrives we calculate the average queue length:
avg = (1-W)*avg + W*current_queue_len,
W is the filter time constant (chosen as 2^(-Wlog)), it controls
the inertia of the algorithm. To allow larger bursts, W should be
decreased.
if (avg > th_max) -> packet marked (dropped).
if (avg < th_min) -> packet passes.
if (th_min < avg < th_max) we calculate probability:
Pb = max_P * (avg - th_min)/(th_max-th_min)
and mark (drop) packet with this probability.
Pb changes from 0 (at avg==th_min) to max_P (avg==th_max).
max_P should be small (not 1), usually 0.01..0.02 is good value.
max_P is chosen as a number, so that max_P/(th_max-th_min)
is a negative power of two in order arithmetics to contain
only shifts.
Parameters, settable by user:
-----------------------------
qth_min - bytes (should be < qth_max/2)
qth_max - bytes (should be at least 2*qth_min and less limit)
Wlog - bits (<32) log(1/W).
Plog - bits (<32)
Plog is related to max_P by formula:
max_P = (qth_max-qth_min)/2^Plog;
F.e. if qth_max=128K and qth_min=32K, then Plog=22
corresponds to max_P=0.02
Scell_log
Stab
Lookup table for log((1-W)^(t/t_ave).
NOTES:
Upper bound on W.
-----------------
If you want to allow bursts of L packets of size S,
you should choose W:
L + 1 - th_min/S < (1-(1-W)^L)/W
th_min/S = 32 th_min/S = 4
log(W) L
-1 33
-2 35
-3 39
-4 46
-5 57
-6 75
-7 101
-8 135
-9 190
etc.
*/
#define RED_STAB_SIZE 256
#define RED_STAB_MASK (RED_STAB_SIZE - 1)
struct red_stats {
u32 prob_drop; /* Early probability drops */
u32 prob_mark; /* Early probability marks */
u32 forced_drop; /* Forced drops, qavg > max_thresh */
u32 forced_mark; /* Forced marks, qavg > max_thresh */
u32 pdrop; /* Drops due to queue limits */
u32 other; /* Drops due to drop() calls */
};
struct red_parms {
/* Parameters */
u32 qth_min; /* Min avg length threshold: A scaled */
u32 qth_max; /* Max avg length threshold: A scaled */
u32 Scell_max;
u32 Rmask; /* Cached random mask, see red_rmask */
u8 Scell_log;
u8 Wlog; /* log(W) */
u8 Plog; /* random number bits */
u8 Stab[RED_STAB_SIZE];
/* Variables */
int qcount; /* Number of packets since last random
number generation */
u32 qR; /* Cached random number */
unsigned long qavg; /* Average queue length: A scaled */
ktime_t qidlestart; /* Start of current idle period */
};
static inline u32 red_rmask(u8 Plog)
{
return Plog < 32 ? ((1 << Plog) - 1) : ~0UL;
}
static inline void red_set_parms(struct red_parms *p,
u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog,
u8 Scell_log, u8 *stab)
{
/* Reset average queue length, the value is strictly bound
* to the parameters below, reseting hurts a bit but leaving
* it might result in an unreasonable qavg for a while. --TGR
*/
p->qavg = 0;
p->qcount = -1;
p->qth_min = qth_min << Wlog;
p->qth_max = qth_max << Wlog;
p->Wlog = Wlog;
p->Plog = Plog;
p->Rmask = red_rmask(Plog);
p->Scell_log = Scell_log;
p->Scell_max = (255 << Scell_log);
memcpy(p->Stab, stab, sizeof(p->Stab));
}
static inline int red_is_idling(struct red_parms *p)
{
return p->qidlestart.tv64 != 0;
}
static inline void red_start_of_idle_period(struct red_parms *p)
{
p->qidlestart = ktime_get();
}
static inline void red_end_of_idle_period(struct red_parms *p)
{
p->qidlestart.tv64 = 0;
}
static inline void red_restart(struct red_parms *p)
{
red_end_of_idle_period(p);
p->qavg = 0;
p->qcount = -1;
}
static inline unsigned long red_calc_qavg_from_idle_time(struct red_parms *p)
{
s64 delta = ktime_us_delta(ktime_get(), p->qidlestart);
long us_idle = min_t(s64, delta, p->Scell_max);
int shift;
/*
* The problem: ideally, average length queue recalcultion should
* be done over constant clock intervals. This is too expensive, so
* that the calculation is driven by outgoing packets.
* When the queue is idle we have to model this clock by hand.
*
* SF+VJ proposed to "generate":
*
* m = idletime / (average_pkt_size / bandwidth)
*
* dummy packets as a burst after idle time, i.e.
*
* p->qavg *= (1-W)^m
*
* This is an apparently overcomplicated solution (f.e. we have to
* precompute a table to make this calculation in reasonable time)
* I believe that a simpler model may be used here,
* but it is field for experiments.
*/
shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK];
if (shift)
return p->qavg >> shift;
else {
/* Approximate initial part of exponent with linear function:
*
* (1-W)^m ~= 1-mW + ...
*
* Seems, it is the best solution to
* problem of too coarse exponent tabulation.
*/
us_idle = (p->qavg * (u64)us_idle) >> p->Scell_log;
if (us_idle < (p->qavg >> 1))
return p->qavg - us_idle;
else
return p->qavg >> 1;
}
}
static inline unsigned long red_calc_qavg_no_idle_time(struct red_parms *p,
unsigned int backlog)
{
/*
* NOTE: p->qavg is fixed point number with point at Wlog.
* The formula below is equvalent to floating point
* version:
*
* qavg = qavg*(1-W) + backlog*W;
*
* --ANK (980924)
*/
return p->qavg + (backlog - (p->qavg >> p->Wlog));
}
static inline unsigned long red_calc_qavg(struct red_parms *p,
unsigned int backlog)
{
if (!red_is_idling(p))
return red_calc_qavg_no_idle_time(p, backlog);
else
return red_calc_qavg_from_idle_time(p);
}
static inline u32 red_random(struct red_parms *p)
{
return net_random() & p->Rmask;
}
static inline int red_mark_probability(struct red_parms *p, unsigned long qavg)
{
/* The formula used below causes questions.
OK. qR is random number in the interval 0..Rmask
i.e. 0..(2^Plog). If we used floating point
arithmetics, it would be: (2^Plog)*rnd_num,
where rnd_num is less 1.
Taking into account, that qavg have fixed
point at Wlog, and Plog is related to max_P by
max_P = (qth_max-qth_min)/2^Plog; two lines
below have the following floating point equivalent:
max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount
Any questions? --ANK (980924)
*/
return !(((qavg - p->qth_min) >> p->Wlog) * p->qcount < p->qR);
}
enum {
RED_BELOW_MIN_THRESH,
RED_BETWEEN_TRESH,
RED_ABOVE_MAX_TRESH,
};
static inline int red_cmp_thresh(struct red_parms *p, unsigned long qavg)
{
if (qavg < p->qth_min)
return RED_BELOW_MIN_THRESH;
else if (qavg >= p->qth_max)
return RED_ABOVE_MAX_TRESH;
else
return RED_BETWEEN_TRESH;
}
enum {
RED_DONT_MARK,
RED_PROB_MARK,
RED_HARD_MARK,
};
static inline int red_action(struct red_parms *p, unsigned long qavg)
{
switch (red_cmp_thresh(p, qavg)) {
case RED_BELOW_MIN_THRESH:
p->qcount = -1;
return RED_DONT_MARK;
case RED_BETWEEN_TRESH:
if (++p->qcount) {
if (red_mark_probability(p, qavg)) {
p->qcount = 0;
p->qR = red_random(p);
return RED_PROB_MARK;
}
} else
p->qR = red_random(p);
return RED_DONT_MARK;
case RED_ABOVE_MAX_TRESH:
p->qcount = -1;
return RED_HARD_MARK;
}
BUG();
return RED_DONT_MARK;
}
#endif