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

[Peter Zijlstra]

On Tue, Jan 30, 2018 at 02:04:32PM +0100, Peter Zijlstra wrote:
> 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

To clarify; my GCC at the time generated conditional branches to compute
the absolute value; and in that case the thing I proposed wins hands
down because its unconditional.

However the above is also unconditional and then the difference is much
less important.

> 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 ;-)