On Fri, Oct 26, 2012 at 1:49 AM, Peter Zijlstra <pet...@infradead.org> wrote:
>
> No, it does a compare on two u128
Actually, it apparently compares two multiplications.
That might be optimizable in itself.
> The point is (as mentioned in the comments below) overflowing an actual
> u64 is rare, however since some of this (specifically the
> dl_{runtime,deadline} parameters) is user specified, we have to assume
> we will overflow.
Any chance we could just limit them?
> + u128 left, right;
> +
> + /*
> + * left and right are the two sides of the equation above,
> + * after a bit of shuffling to use multiplications instead
> + * of divisions.
> + *
> + * Note that none of the time values involved in the two
> + * multiplications are absolute: dl_deadline and dl_runtime
> + * are the relative deadline and the maximum runtime of each
> + * instance, runtime is the runtime left for the last instance
> + * and (deadline - t), since t is rq->clock, is the time left
> + * to the (absolute) deadline. Therefore, overflowing the u64
> + * type is very unlikely to occur in both cases.
> + */
> + left = mul_u64_u64(dl_se->dl_deadline, dl_se->runtime);
> + right = mul_u64_u64((dl_se->deadline - t), dl_se->dl_runtime);
> +
> + if (cmp_u128(left, right) > 0)
> + return true;
> +
> + return false;
So how often could we do this without doing the multiplication at all?
It's trivial to see that 'right > left' if the individual
multiplicands are both bigger, for example. Maybe that is common?
And even if it overflows in 64-bit does it overflow in 92? For 32-bit
machines, the difference there is quite noticeable.
So the above might actually be better written as a
"compare_64bit_multiply(a,b,c,d)". At the same time, are we
*seriously* ever talking about multi-second runtimes or deadlines?
Because even in nanoseconds, I assume that the common case *by*far* in
scheduling would be about values smaller than four seconds, in which
case all of the above values are 32-bit, making the compares *much*
cheaper.
So on a 32-bit machine (say, x86-32), you might just have:
- or all the high words together, jump to slow case if the result is non-zero
- otherwise, do just two 32x32 multiplies and check which of the two is bigger.
That's a *huge* reduction in expensive multiplications.
And *THAT* is why generic 128-bit math is stupid. Don't do it.
Linus
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