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

From: Peter Zijlstra
Date: Tue Jan 30 2018 - 09:01:42 EST


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 ;-)