netfilter: conntrack: refine gc worker heuristics

netfilter: conntrack: refine gc worker heuristics

Nicolas Dichtel says:
  After commit b87a2f9199ea ("netfilter: conntrack: add gc worker to
  remove timed-out entries"), netlink conntrack deletion events may be
  sent with a huge delay.

Nicolas further points at this line:

  goal = min(nf_conntrack_htable_size / GC_MAX_BUCKETS_DIV, GC_MAX_BUCKETS);

and indeed, this isn't optimal at all.  Rationale here was to ensure that
we don't block other work items for too long, even if
nf_conntrack_htable_size is huge.  But in order to have some guarantee
about maximum time period where a scan of the full conntrack table
completes we should always use a fixed slice size, so that once every
N scans the full table has been examined at least once.

We also need to balance this vs. the case where the system is either idle
(i.e., conntrack table (almost) empty) or very busy (i.e. eviction happens
from packet path).

So, after some discussion with Nicolas:

1. want hard guarantee that we scan entire table at least once every X s
-> need to scan fraction of table (get rid of upper bound)

2. don't want to eat cycles on idle or very busy system
-> increase interval if we did not evict any entries

3. don't want to block other worker items for too long
-> make fraction really small, and prefer small scan interval instead

4. Want reasonable short time where we detect timed-out entry when
system went idle after a burst of traffic, while not doing scans
all the time.
-> Store next gc scan in worker, increasing delays when no eviction
happened and shrinking delay when we see timed out entries.

The old gc interval is turned into a max number, scans can now happen
every jiffy if stale entries are present.

Longest possible time period until an entry is evicted is now 2 minutes
in worst case (entry expires right after it was deemed 'not expired').

Reported-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>