linux-kernel - Re: [PATCH] random: get_random_u64

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Message-ID: <2ekh7esfcxnqdnjxufybel5kkpazjvwadwnb7zok5ske4vmptr@573764kocoga>
Date: Sat, 15 Mar 2025 17:32:38 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: David Laight <david.laight.linux@...il.com>
Cc: linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>, 
	"Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH] random: get_random_u64_below()

On Sat, Mar 15, 2025 at 08:55:32PM +0000, David Laight wrote:
> On Sat, 15 Mar 2025 14:20:46 -0400
> Kent Overstreet <kent.overstreet@...ux.dev> wrote:
> 
> > 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@...ux.dev> 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.
> 
> I make it two multiplies and a loop.
> Have you looked at what happens on 32bit systems?

Not many people run 32 bit anymore.

Have you looked at get_random_bytes()?

> > 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.
> 
> If the % is only a small fraction on the cost for the non-inline case
> (and it is over 100 clocks on many cpu that people still use) then
> why inline anything?
> A quick look shows divide being 'only moderately slow' on zen3 and coffee lake.
> 
> What you might want to do is pass -ceil % ceil through to a real function
> (especially if constant).

David, the way get_random_u32_below is optimized now is just fine.

I'm not going to completely redo the inlining just for this, nor am I
going to do get_random_u64_below() differently from
get_random_u32_below().

> Oh I guess you haven't actually tested the version you submitted.
> Time to play 'spot the silly error'.

Please do share what you think you've found.