Subject: Re: [PATCH] bpf: call get_random_u32() for random integers

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 06 Dec 2022 14:53:22 +0100

"Jason A. Donenfeld" <[email-masked]> writes:

> Hi Toke,
>
> On Tue, Dec 6, 2022 at 2:26 PM Toke Høiland-Jørgensen <[email-masked]> wrote:
>> So for instance, if there's a large fixed component of the overhead of
>> get_random_u32(), we could have bpf_user_rnd_u32() populate a larger
>> per-CPU buffer and then just emit u32 chunks of that as long as we're
>> still in the same NAPI loop as the first call. Or something to that
>> effect. Not sure if this makes sense for this use case, but figured I'd
>> throw the idea out there :)
>
> Actually, this already is how get_random_u32() works! It buffers a
> bunch of u32s in percpu batches, and doles them out as requested.

Ah, right. Not terribly surprised you already did this!

> However, this API currently works in all contexts, including in
> interrupts. So each call results in disabling irqs and reenabling
> them. If I bifurcated batches into irq batches and non-irq batches, so
> that we only needed to disable preemption for the non-irq batches,
> that'd probably improve things quite a bit, since then the overhead
> really would reduce to just a memcpy for the majority of calls. But I
> don't know if adding that duplication of all code paths is really
> worth the huge hassle.

Right, makes sense; happy to leave that decision entirely up to you :)

-Toke