Re: [PATCH net 2/2] conntrack: enable to tune gc parameters

Le 13/10/2016 à 22:43, Florian Westphal a écrit :

Nicolas Dichtel [off-list ref] wrote:

quoted

Le 10/10/2016 à 16:04, Florian Westphal a écrit :

quoted

Nicolas Dichtel [off-list ref] wrote:

quoted

After commit b87a2f9199ea ("netfilter: conntrack: add gc worker to remove
timed-out entries"), netlink conntrack deletion events may be sent with a
huge delay. It could be interesting to let the user tweak gc parameters
depending on its use case.

Hmm, care to elaborate?

I am not against doing this but I'd like to hear/read your use case.

The expectation is that in almot all cases eviction will happen from
packet path.  The gc worker is jusdt there for case where a busy system
goes idle.

It was precisely that case. After a period of activity, the event is sent a long
time after the timeout. If the router does not manage a lot of flows, why not
trying to parse more entries instead of the default 1/64 of the table?
In fact, I don't understand why using GC_MAX_BUCKETS_DIV instead of using always
GC_MAX_BUCKETS whatever the size of the table is.

I wanted to make sure that we have a known upper bound on the number of
buckets we process so that we do not block other pending kworker items
for too long.

I don't understand. GC_MAX_BUCKETS is the upper bound and I agree that it is
needed. But why GC_MAX_BUCKETS_DIV (ie 1/64)?
In other words, why this line:
goal = min(nf_conntrack_htable_size / GC_MAX_BUCKETS_DIV, GC_MAX_BUCKETS);
instead of:
goal = GC_MAX_BUCKETS;
?

(Or cause too many useless scans)

Another idea worth trying might be to get rid of the max cap and
instead break early in case too many jiffies expired.

I don't want to add sysctl knobs for this unless absolutely needed; its already
possible to 'force' eviction cycle by running 'conntrack -L'.

Sure, but this is not a "real" solution, just a workaround.
We need to find a way to deliver conntrack deletion events in a reasonable
delay, whatever the traffic on the machine is.