linux-kernel - Re: [PATCH] sched/cputime: make scale

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Date:   Mon, 22 Jul 2019 21:45:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Fox <afox@...hat.com>,
        Stephen Johnston <sjohnsto@...hat.com>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Stanislaw Gruszka <sgruszka@...hat.com>
Subject: Re: [PATCH] sched/cputime: make scale_stime() more precise

On Fri, Jul 19, 2019 at 04:03:25PM +0200, Oleg Nesterov wrote:
> On 07/19, Peter Zijlstra wrote:
> > Included below is also an x86_64 implementation in 2 instructions.
> 
> But we need the arch-neutral implementation anyway, the code above
> is the best I could invent.

Agreed; we do. Depending on the cost of division and if the arch has a
64x64->128 mult, it might be better to compute a reciprocal and multiply
that, but yes, long staring didn't get me much better ideas either.

> But see below!
> 
> > I'm still trying see if there's anything saner we can do...
> 
> Oh, please, it is not that I like my solution very much, I would like
> to see something more clever.
> 
> > static noinline u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
> > {
> > 	u64 q;
> > 	asm ("mulq %2; divq %3" : "=a" (q) : "a" (a), "rm" (b), "rm" (c) : "rdx");
> > 	return q;
> > }
> 
> Heh. I have to admit that I didn't know that divq divides 128bit by
> 64bit. gcc calls the __udivti3 intrinsic in this case so I wrongly
> came to conclusion this is not simple even on x86_64. Plus the fact
> that linux/math64.h only has mul_u64_u64_shr()...

C wants to promote the dividend and divisor to the same type (int128)
and then it runs into trouble.

But yeah, I don't know how many other 64bit archs can pull off that
trick. I asked, and ARGH64 cannot do that 128/64 (although it can do a
64x64->128 in two instructions).