Re: [PATCH v3 1/3] sched/fair: add util_est on top of PELT

[Peter Zijlstra]

On Tue, Jan 30, 2018 at 12:46:33PM +0000, Patrick Bellasi wrote:
> > Aside from that being whitespace challenged, did you also try:
> > 
> > 	if ((unsigned)((util_est - util_last) + LIM - 1) < (2 * LIM - 1))
> 
> No, since the above code IMO is so much "easy to parse for humans" :)

Heh, true. Although that's fixable by wrapping it in some helper with a
comment.

> But, mainly because since the cache alignment update, also while testing on a
> "big" Intel machine I cannot see regressions on hackbench.
> 
> This is the code I get on my Xeon E5-2690 v2:
> 
>        if (abs(util_est - util_last) <= (SCHED_CAPACITY_SCALE / 100))
>    6ba0:       8b 86 7c 02 00 00       mov    0x27c(%rsi),%eax
>    6ba6:       48 29 c8                sub    %rcx,%rax
>    6ba9:       48 99                   cqto
>    6bab:       48 31 d0                xor    %rdx,%rax
>    6bae:       48 29 d0                sub    %rdx,%rax
>    6bb1:       48 83 f8 0a             cmp    $0xa,%rax
>    6bb5:       7e 1d                   jle    6bd4 <dequeue_task_fair+0x7e4>
> 
> Does it look so bad?

Its not terrible, and I think your GCC is far more clever than the one I
used at the time. But that's 4 dependent instructions (cqto,xor,sub,cmp)
whereas the one I proposed uses only 2 (add,cmp).

Now, my proposal is, as you say, somewhat hard to read, and it also
doesn't work right when our values are 'big' (which they will not be in
our case, because util has a very definite bound), and I suspect you're
right that ~2 cycles here will not be measurable.

So yeah.... whatever ;-)