`On Tue, Mar 12, 2019 at 03:50:43PM +0000, Patrick Bellasi wrote:> On 12-Mar 16:20, Peter Zijlstra wrote:> > On Fri, Feb 08, 2019 at 10:05:40AM +0000, Patrick Bellasi wrote:> > > +/* Integer ceil-rounded range for each bucket */          ^^^^^^^^^^^^^^^^^^^^^^^^^^^> > > +#define UCLAMP_BUCKET_DELTA ((SCHED_CAPACITY_SCALE / UCLAMP_BUCKETS) + 1)                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^simply do not match.> > Uhm, should that not me ((x+y-1)/y), aka. DIV_ROUND_UP(x,y) ?> > Well, there is certainly some rounding to be done...> > > The above would give 4 for 9/3, which is clearly buggered.> > .. still the math above should work fine within the boundaries we> define for UCLAMP_BUCKET_DELTA (5..20 groups) and considering that> SCHED_CAPACITY_SCALE will never be smaller then 1024.That's a very poor reason to write utter nonsense :-)> The above is designed to shrink the topmost bucket wrt all the others> but it will never be smaller than ~30%.30% sounds like a lot, esp. for this range.> Here are the start values computed for each bucket using the math> above and the computed shrinking percentage for the topmost bucket:If you use a regular rounding, the error is _much_ smaller:\$ for ((x=5;x<21;x++)) ; do let d=(1024+x/2)/x; let s=(x-1)*d; let e=1024-s; let p=100*(d-e)/d; echo \$x \$d \$s \$e \$p%; done5 205 820 204 0%6 171 855 169 1%7 146 876 148 -1%8 128 896 128 0%9 114 912 112 1%10 102 918 106 -3%11 93 930 94 -1%12 85 935 89 -4%13 79 948 76 3%14 73 949 75 -2%15 68 952 72 -5%16 64 960 64 0%17 60 960 64 -6%18 57 969 55 3%19 54 972 52 3%20 51 969 55 -7%Funnily enough, we have a helper for that too: DIV_ROUND_CLOSEST().Now, if we go further, the error will obviously increase because we runout of precision, but even there, regular rounding will be better thaneither floor or ceil.`