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

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Message-ID: <f8f2d73a-4801-fd49-b028-7a643f61ab24@wanadoo.fr>
Date:   Mon, 1 Aug 2022 20:01:45 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     David Laight <David.Laight@...LAB.COM>,
        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()

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
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
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