From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Andrew Fox <afox@redhat.com>,
Stephen Johnston <sjohnsto@redhat.com>,
linux-kernel@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
Stanislaw Gruszka <sgruszka@redhat.com>
Subject: Re: [PATCH] sched/cputime: make scale_stime() more precise
Date: Mon, 22 Jul 2019 21:45:24 +0200 [thread overview]
Message-ID: <20190722194524.GH6698@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20190719140325.GA31938@redhat.com>
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).
next prev parent reply other threads:[~2019-07-22 19:45 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-18 13:18 [PATCH] sched/cputime: make scale_stime() more precise Oleg Nesterov
2019-07-18 13:21 ` Oleg Nesterov
2019-07-18 14:55 ` Oleg Nesterov
2019-07-19 11:03 ` Peter Zijlstra
2019-07-19 13:47 ` Peter Zijlstra
2019-07-19 14:37 ` Oleg Nesterov
2019-07-22 19:56 ` Peter Zijlstra
2019-07-23 14:00 ` Oleg Nesterov
2019-07-23 14:29 ` Oleg Nesterov
2019-07-19 14:03 ` Oleg Nesterov
2019-07-22 19:45 ` Peter Zijlstra [this message]
2019-07-22 10:52 ` Stanislaw Gruszka
2019-07-22 20:00 ` Peter Zijlstra
2019-07-23 9:37 ` Stanislaw Gruszka
2020-01-22 16:46 ` Oleg Nesterov
2020-01-23 13:05 ` Oleg Nesterov
2020-01-24 15:42 ` Oleg Nesterov
2020-01-27 12:28 ` [PATCH v2] " Oleg Nesterov
2020-05-15 17:24 ` Oleg Nesterov
2020-05-19 17:25 ` Peter Zijlstra
2020-05-19 18:33 ` Linus Torvalds
2020-05-19 18:42 ` Peter Zijlstra
2020-05-19 19:11 ` Peter Zijlstra
2020-05-19 19:51 ` Linus Torvalds
2020-05-20 15:24 ` Oleg Nesterov
2020-05-20 15:36 ` Peter Zijlstra
2020-05-20 20:10 ` Peter Zijlstra
2020-05-21 13:26 ` Oleg Nesterov
2020-06-16 12:21 ` [tip: sched/core] sched/cputime: Improve cputime_adjust() tip-bot2 for Oleg Nesterov
-- strict thread matches above, loose matches on Subject: below --
2019-07-18 13:15 [PATCH] sched/cputime: make scale_stime() more precise Oleg Nesterov
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