netdev - Re: [PATCH 09/14] netfilter: conntrack: refine gc worker heuristics, redux

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Date:   Fri, 27 Jan 2017 17:51:11 +0100
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     Pablo Neira Ayuso <pablo@...filter.org>,
        netfilter-devel@...r.kernel.org
Cc:     davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH 09/14] netfilter: conntrack: refine gc worker heuristics,
 redux

Le 26/01/2017 à 17:38, Pablo Neira Ayuso a écrit :
> From: Florian Westphal <fw@...len.de>
> 
> This further refines the changes made to conntrack gc_worker in
> commit e0df8cae6c16 ("netfilter: conntrack: refine gc worker heuristics").
> 
> The main idea of that change was to reduce the scan interval when evictions
> take place.
> 
> However, on the reporters' setup, there are 1-2 million conntrack entries
> in total and roughly 8k new (and closing) connections per second.
> 
> In this case we'll always evict at least one entry per gc cycle and scan
> interval is always at 1 jiffy because of this test:
> 
>  } else if (expired_count) {
>      gc_work->next_gc_run /= 2U;
>      next_run = msecs_to_jiffies(1);
> 
> being true almost all the time.
> 
> Given we scan ~10k entries per run its clearly wrong to reduce interval
> based on nonzero eviction count, it will only waste cpu cycles since a vast
> majorities of conntracks are not timed out.
> 
> Thus only look at the ratio (scanned entries vs. evicted entries) to make
> a decision on whether to reduce or not.
> 
> Because evictor is supposed to only kick in when system turns idle after
> a busy period, pick a high ratio -- this makes it 50%.  We thus keep
> the idea of increasing scan rate when its likely that table contains many
> expired entries.
> 
> In order to not let timed-out entries hang around for too long
> (important when using event logging, in which case we want to timely
> destroy events), we now scan the full table within at most
> GC_MAX_SCAN_JIFFIES (16 seconds) even in worst-case scenario where all
> timed-out entries sit in same slot.
> 
> I tested this with a vm under synflood (with
> sysctl net.netfilter.nf_conntrack_tcp_timeout_syn_recv=3).
> 
> While flood is ongoing, interval now stays at its max rate
> (GC_MAX_SCAN_JIFFIES / GC_MAX_BUCKETS_DIV -> 125ms).
> 
> With feedback from Nicolas Dichtel.
> 
> Reported-by: Denys Fedoryshchenko <nuclearcat@...learcat.com>
> Cc: Nicolas Dichtel <nicolas.dichtel@...nd.com>
> Fixes: b87a2f9199ea82eaadc ("netfilter: conntrack: add gc worker to remove timed-out entries")
> Signed-off-by: Florian Westphal <fw@...len.de>
> Tested-by: Nicolas Dichtel <nicolas.dichtel@...nd.com>
> Acked-by: Nicolas Dichtel <nicolas.dichtel@...nd.com>
> Tested-by: Denys Fedoryshchenko <nuclearcat@...learcat.com>
> Signed-off-by: Pablo Neira Ayuso <pablo@...filter.org>
Pablo, is it possible to queue this patch (and the previous: 08/14) for the 4.9
stable?


Thank you,
Nicolas