On 10 September 2015 at 13:06, Morten Rasmussen <morten.rasmus...@arm.com> wrote: > On Wed, Sep 09, 2015 at 03:23:43PM -0700, bseg...@google.com wrote: >> Peter Zijlstra <pet...@infradead.org> writes: >> >> > On Tue, Sep 08, 2015 at 03:31:58PM +0100, Morten Rasmussen wrote: >> >> On Tue, Sep 08, 2015 at 02:52:05PM +0200, Peter Zijlstra wrote: >> >> > > Tricky that, LOAD_AVG_MAX very much relies on the unit being 1<<10. >> >> >> >> I don't get why LOAD_AVG_MAX relies on the util_avg shifting being >> >> 1<<10, it is just the sum of the geometric series and the upper bound of >> >> util_sum? >> > >> > It needs a 1024, it might just have been the 1024 ns we use a period >> > instead of the scale unit though. >> > >> > The LOAD_AVG_MAX is the number where adding a next element to the series >> > doesn't change the result anymore, so scaling it up will allow more >> > significant elements to the series before we bottom out, which is the _N >> > thing. >> > >> >> Yes, as the comments say, the 1024ns unit is arbitrary (and is an >> average of not-quite-microseconds instead of just nanoseconds to allow >> more bits to load.weight when we multiply load.weight by this number). >> In fact there are two arbitrary 1024 units here, which are technically >> unrelated and are both unrelated to SCHED_LOAD_RESOLUTION/etc - we >> operate on units of almost-microseconds and we also do decays every >> almost-millisecond. >> >> There appears to be a bunch of confusion in the current code around >> util_sum/util_avg which appears to using SCHED_LOAD_SCALE >> for a fixed-point percentage or something, which is at least reasonable, >> but is initializing it as scale_load_down(SCHED_LOAD_SCALE), which >> results in either initializing as 100% or .1% depending on RESOLUTION. >> This'll get clobbered on first update, but if it needs to be >> initialized, it should either get initialized to something sane or at >> least consistent. > > This is what I thought too. The whole geometric series math is completely > independent of the scale used for priority in load_avg and the fixed > point shifting used for util_avg. > >> load_sum/load_avg appear to be scale_load_down()ed properly, and appear >> to be used as such at a quick glance. > > I don't think shifting by SCHED_LOAD_SHIFT in __update_load_avg() is > right: > > sa->util_avg = (sa->util_sum << SCHED_LOAD_SHIFT) / LOAD_AVG_MAX; > > util_avg is initialized to low resolution (>> SCHED_LOAD_RESOLUTION): > > sa->util_avg = scale_load_down(SCHED_LOAD_SCALE); > > so it appear to be intended to be using low resolution like load_avg > (weight is scaled down before it is passed into __update_load_avg()), > but util_avg is shifted up to high resolution. It should be: > > sa->util_avg = (sa->util_sum << (SCHED_LOAD_SHIFT - > SCHED_LOAD_SHIFT)) / LOAD_AVG_MAX;
you probably mean (SCHED_LOAD_SHIFT - SCHED_LOAD_RESOLUTION) The goal of this patchset is to be able to scale util_avg in the range of cpu capacity so why don't we directly initialize it with sa->util_avg = SCHED_CAPACITY_SCALE; and then use sa->util_avg = (sa->util_sum << SCHED_CAPACITY_SHIFT) / LOAD_AVG_MAX; so we don't have to take care of high and low load resolution Regards, > > to be consistent. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
