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Date:   Tue, 29 Sep 2020 19:05:24 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Daniel Borkmann <daniel@...earbox.net>
CC:     <ast@...nel.org>, <john.fastabend@...il.com>,
        <netdev@...r.kernel.org>, <bpf@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH bpf-next v3 2/6] bpf, net: rework cookie generator as
 per-cpu one

On Tue, Sep 29, 2020 at 11:23:02PM +0200, Daniel Borkmann wrote:
> With its use in BPF, the cookie generator can be called very frequently
> in particular when used out of cgroup v2 hooks (e.g. connect / sendmsg)
> and attached to the root cgroup, for example, when used in v1/v2 mixed
> environments. In particular, when there's a high churn on sockets in the
> system there can be many parallel requests to the bpf_get_socket_cookie()
> and bpf_get_netns_cookie() helpers which then cause contention on the
> atomic counter.
> 
> As similarly done in f991bd2e1421 ("fs: introduce a per-cpu last_ino
> allocator"), add a small helper library that both can use for the 64 bit
> counters. Given this can be called from different contexts, we also need
> to deal with potential nested calls even though in practice they are
> considered extremely rare. One idea as suggested by Eric Dumazet was
> to use a reverse counter for this situation since we don't expect 64 bit
> overflows anyways; that way, we can avoid bigger gaps in the 64 bit
> counter space compared to just batch-wise increase. Even on machines
> with small number of cores (e.g. 4) the cookie generation shrinks from
> min/max/med/avg (ns) of 22/50/40/38.9 down to 10/35/14/17.3 when run
> in parallel from multiple CPUs.
Acked-by: Martin KaFai Lau <kafai@...com>

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