fq_codel: add batch ability to fq_codel_drop()

In presence of inelastic flows and stress, we can call
fq_codel_drop() for every packet entering fq_codel qdisc.

fq_codel_drop() is quite expensive, as it does a linear scan
of 4 KB of memory to find a fat flow.
Once found, it drops the oldest packet of this flow.

Instead of dropping a single packet, try to drop 50% of the backlog
of this fat flow, with a configurable limit of 64 packets per round.

TCA_FQ_CODEL_DROP_BATCH_SIZE is the new attribute to make this
limit configurable.

With this strategy the 4 KB search is amortized to a single cache line
per drop [1], so fq_codel_drop() no longer appears at the top of kernel
profile in presence of few inelastic flows.

[1] Assuming a 64byte cache line, and 1024 buckets

Signed-off-by: Eric Dumazet <[email protected]>
Reported-by: Dave Taht <[email protected]>
Cc: Jonathan Morton <[email protected]>
Acked-by: Jesper Dangaard Brouer <[email protected]>
Acked-by: Dave Taht
Signed-off-by: David S. Miller <[email protected]>

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