linux-kernel - RE: [PATCH v2] timers: Optimize usleep

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Date:   Mon, 1 Aug 2022 08:18:26 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christophe JAILLET' <christophe.jaillet@...adoo.fr>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>
Subject: RE: [PATCH v2] timers: Optimize usleep_range()

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?
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.

	David

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