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 ;-)
