In make_eligible, a mask is used to decide which groups must become eligible: the i-th group becomes eligible only if the i-th bit of the mask (from the right) is set. The mask is computed by left-shifting a 1 by a given number of places, and decrementing the result. The shift is performed on a ULL to avoid problems in case the number of places to shift is higher than 31. On a 32-bit machine, this is more costly than working on an UL. This patch replaces such a costly operation with two cheaper branches.
The trick is based on the following fact: in case of a shift of at least 32 places, the resulting mask has at least the 32 less significant bits set, whereas the total number of groups is lower than 32. As a consequence, in this case it is enough to just set the 32 less significant bits of the mask with a cheaper ~0UL. In the other case, the shift can be safely performed on a UL. Reported-by: David S. Miller <da...@davemloft.net> Reported-by: David Laight <david.lai...@aculab.com> Signed-off-by: Paolo Valente <paolo.vale...@unimore.it> --- net/sched/sch_qfq.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/net/sched/sch_qfq.c b/net/sched/sch_qfq.c index d51852b..2a0ed50 100644 --- a/net/sched/sch_qfq.c +++ b/net/sched/sch_qfq.c @@ -821,7 +821,14 @@ static void qfq_make_eligible(struct qfq_sched *q) unsigned long old_vslot = q->oldV >> q->min_slot_shift; if (vslot != old_vslot) { - unsigned long mask = (1ULL << fls(vslot ^ old_vslot)) - 1; + unsigned long mask; + int last_flip_pos = fls(vslot ^ old_vslot); + + if (last_flip_pos > 31) /* higher than the number of groups */ + mask = ~0UL; /* make all groups eligible */ + else + mask = (1UL << last_flip_pos) - 1; + qfq_move_groups(q, mask, IR, ER); qfq_move_groups(q, mask, IB, EB); } -- 1.8.1.2 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/