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References: <20221205181534.612702-1-Jason@zx2c4.com> <730fd355-ad86-a8fa-6583-df23d39e0c23@iogearbox.net>
 <Y451ENAK7BQQDJc/@zx2c4.com> <87lenku265.fsf@toke.dk>
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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Tue, 6 Dec 2022 13:59:31 +0100
Message-ID: <CAHmME9poicgpHhXJ1ieWbDTFBu=ApSFaQKShhHezDmA0A5ajKQ@mail.gmail.com>
Subject: Re: [PATCH] bpf: call get_random_u32() for random integers
To:     =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@kernel.org>
Cc:     Daniel Borkmann <daniel@iogearbox.net>, bpf@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>
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Hi Toke,

On Tue, Dec 6, 2022 at 1:50 PM Toke H=C3=B8iland-J=C3=B8rgensen <toke@kerne=
l.org> wrote:
>
> "Jason A. Donenfeld" <Jason@zx2c4.com> writes:
>
> > On Mon, Dec 05, 2022 at 11:21:51PM +0100, Daniel Borkmann wrote:
> >> On 12/5/22 7:15 PM, Jason A. Donenfeld wrote:
> >> > Since BPF's bpf_user_rnd_u32() was introduced, there have been three
> >> > significant developments in the RNG: 1) get_random_u32() returns the
> >> > same types of bytes as /dev/urandom, eliminating the distinction bet=
ween
> >> > "kernel random bytes" and "userspace random bytes", 2) get_random_u3=
2()
> >> > operates mostly locklessly over percpu state, 3) get_random_u32() ha=
s
> >> > become quite fast.
> >>
> >> Wrt "quite fast", do you have a comparison between the two? Asking as =
its
> >> often used in networking worst case on per packet basis (e.g. via XDP)=
, would
> >> be useful to state concrete numbers for the two on a given machine.
> >
> > Median of 25 cycles vs median of 38, on my Tiger Lake machine. So a
> > little slower, but too small of a difference to matter.
>
> Assuming a 3Ghz CPU clock (so 3 cycles per nanosecond), that's an
> additional overhead of ~4.3 ns. When processing 10 Gbps at line rate
> with small packets, the per-packet processing budget is 67.2 ns, so
> those extra 4.3 ns will eat up ~6.4% of the budget.
>
> So in other words, "too small a difference to matter" is definitely not
> true in general. It really depends on the use case; if someone is using
> this to, say, draw per-packet random numbers to compute a drop frequency
> on ingress, that extra processing time will most likely result in a
> quite measurable drop in performance.

Huh, neat calculation, I'll keep that method in mind.

Alright, sorry for the noise here. I'll check back in if I ever manage
to eliminate that performance gap.

Jason