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[PKT_SCHED] RED: Fix overflow in calculation of queue average

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Overflow can occur very easily with 32 bits, e.g., with 1 second
us_idle is approx. 2^20, which leaves only 11-Wlog bits for queue
length. Since the EWMA exponent is typically around 9, queue
lengths larger than 2^2 cause overflow. Whether the affected
branch is taken when us_idle is as high as 1 second, depends on
Scell_log, but with rather reasonable configuration Scell_log is
large enough to cause p->Stab to have zero index, which always
results zero shift (typically also few other small indices result
in zero shift).

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Ilpo Järvinen 2006-08-04 16:36:18 -07:00 committed by David S. Miller
parent 30a584d944
commit c4c0ce5c57
1 changed files with 1 additions and 1 deletions
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2
include/net/red.h
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@ -212,7 +212,7 @@ static inline unsigned long red_calc_qavg_from_idle_time(struct red_parms *p)
* Seems, it is the best solution to
* problem of too coarse exponent tabulation.
*/
us_idle = (p->qavg * us_idle) >> p->Scell_log;
us_idle = (p->qavg * (u64)us_idle) >> p->Scell_log;
if (us_idle < (p->qavg >> 1))
return p->qavg - us_idle;