Re: [PATCH v2] timers: Optimize usleep_range()

[Christophe JAILLET]

Le 01/08/2022 à 10:18, David Laight a écrit :
> From: Christophe JAILLET
> > Sent: 29 July 2022 21:29
> >
> > Most of the time the 'min' and 'max' parameters of usleep_range() are
> > constant. We can take advantage of it to pre-compute at compile time
> > some values otherwise computer at run-time in usleep_range_state().
> >
> > Replace usleep_range_state() by a new __nsleep_range_delta_state() function
> > that takes as parameters the pre-computed values.
> >
> > The main benefit is to save a few instructions, especially 2
> > multiplications (x1000 when converting us to ns).
> ...
> >    53                   	push   %rbx
> >    48 89 fb             	mov    %rdi,%rbx
> >    81 e5 cc 00 00 00    	and    $0xcc,%ebp
> > - 49 29 dc             	sub    %rbx,%r12              ; (max - min)
> > - 4d 69 e4 e8 03 00 00 	imul   $0x3e8,%r12,%r12       ; us --> ns (x 1000)
> >    48 83 ec 68          	sub    $0x68,%rsp
> >    48 c7 44 24 08 b3 8a 	movq   $0x41b58ab3,0x8(%rsp)
> >    b5 41
> > @@ -10721,18 +10719,16 @@
> >    31 c0                	xor    %eax,%eax
> >    e8 00 00 00 00       	call   ...
> >    e8 00 00 00 00       	call   ...
> > - 49 89 c0             	mov    %rax,%r8
> > - 48 69 c3 e8 03 00 00 	imul   $0x3e8,%rbx,%rax       ; us --> ns (x 1000)
> > + 48 01 d8             	add    %rbx,%rax
> > + 48 89 44 24 28       	mov    %rax,0x28(%rsp)
> >    65 48 8b 1c 25 00 00 	mov    %gs:0x0,%rbx
> >    00 00
> > - 4c 01 c0             	add    %r8,%rax
> > - 48 89 44 24 28       	mov    %rax,0x28(%rsp)
> >    e8 00 00 00 00       	call   ...
> ...
>
> Is that really measurable in any test?

I don't think so, even on 32 bits arch.

> Integer multiply is one clock on almost every modern cpu.
>
> By the time you've allowed for superscaler cpu there is
> probably no difference at all on anything except the simplest
> cpus.

My point is that it is a low hanging fruit.
Just moving some simple computations from one function to another, to 
have the compiler do the job instead of at runtime.

I won't argue the value of the patch itself.
I spotted a potential opportunity and proposed a patch for it.

If someone finds it valuable enough, just take it.
If no-one care, just forget about it.

Both alternative are fine for me.

Best regards,
CJ

> 	David
>
> -
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