Re: [PATCH] random: get_random_u64_below()

[Kent Overstreet]

On Sat, Mar 15, 2025 at 01:52:34PM +0000, David Laight wrote:
> On Thu, 13 Mar 2025 12:38:10 -0400
> Kent Overstreet <kent.overstreet@xxxxxxxxx> wrote:
> 
> > bcachefs needs this, for sampling devices to read from based on squared
> > device latencies.
> > 
> > this uses the same algorithm as get_random_u32_below: since the multiply
> > uses the top and bottom halves separately, it works out fairly well.
> 
> Adding two separate copies of much the same code is silly.
> Given what the code is doing, does it ever make any sense to inline it.
> 
> Inlining the original get_random_u32_below(ceil) that did
> 	(random_u32() * ((1ull << 32) / ceil) >> 32
> (for constant ceil) made sense.
> While good enough for most purposes it was replaced by the much more
> expensive function that guarantees that all the output values are
> equally likely - rather than just evenly distributed.

Expensive!? It adds a multiply.

That % gets constant folded, in the inlined case, and in the non-inline
case it's hit only a small fraction of the, time, for typical ceil.