Re: questions on calc_delta_mine() in sched.c

Peter Zijlstra
2008-05-02T12:14:26Z

On Thu, 2008-05-01 at 15:45 -0500, Joel Schopp wrote:
> Ingo,
>
> I have a few questions regarding this code in kernel/sched.c
>
> static unsigned long
> calc_delta_mine(unsigned long delta_exec, unsigned long weight,
> struct load_weight *lw)
> {
> u64 tmp;
>
> if (unlikely(!lw->inv_weight))
> lw->inv_weight = (WMULT_CONST-lw->weight/2) / (lw->weight+1);
>
>
> Q1) This code is hit often in scenarios I run, is this really unlikely for
> others?

I think it became a lot more likely recently, perhaps removing that
unlikely is not such a bad idea.

> Q2) The rest of the code in sched.c seems to make inv_weight ==
> WMULT_CONST/weight and I was wondering if you could explain why this
> instance is different.

because the rest of the code is wrong, there are only 2 other sites, and
I have a patch that removes those div64_64() with =0;

The idea is to use rounding division: (x + y/2) / y
but we can't because 'x' is touching the limits of our modulo space,
hence we do: (x - y/2) / y
which comes in 1 short, that fixup has been lost along the way.

> Q3) That division is pretty expensive, could we sacrifice some accuracy and
> do a precompute table? Do you have another idea how we could get rid of
> the divide?

Is a full memory miss not more expensive on most modern machines?

And, no sadly I have no ideas on how to get rid of it ;-/