linux-kernel - Re: [PATCH] bpf: call get_random

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Message-ID: <87lenku265.fsf@toke.dk>
Date:   Tue, 06 Dec 2022 13:50:10 +0100
From:   Toke Høiland-Jørgensen <toke@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>
Subject: Re: [PATCH] bpf: call get_random_u32() for random integers

"Jason A. Donenfeld" <Jason@...c4.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 between
>> > "kernel random bytes" and "userspace random bytes", 2) get_random_u32()
>> > operates mostly locklessly over percpu state, 3) get_random_u32() has
>> > 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.

-Toke