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

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Date:   Fri, 14 Oct 2016 12:12:29 +0200
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     Florian Westphal <fw@...len.de>
Cc:     davem@...emloft.net, pablo@...filter.org, netdev@...r.kernel.org,
        netfilter-devel@...r.kernel.org
Subject: Re: [PATCH net 2/2] conntrack: enable to tune gc parameters

Le 13/10/2016 à 22:43, Florian Westphal a écrit :
> Nicolas Dichtel <nicolas.dichtel@...nd.com> wrote:
>> Le 10/10/2016 à 16:04, Florian Westphal a écrit :
>>> Nicolas Dichtel <nicolas.dichtel@...nd.com> wrote:
>>>> 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.