On Thu, Sep 10, 2015 at 01:11:01PM +0200, Vincent Guittot wrote: > 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)
Yes. Thanks for providing the right expression. There seems to be enough confusion in this thread already :) > 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 That works for me, except that the left-shift has gone be PeterZ's optimization patch posted earlier in this thread. It is changing util_sum to scaled by capacity instead of being the pure geometric series which requires the left shift at the end when we divide by LOAD_AVG_MAX. So it should be equivalent to what you are proposing if we change the initialization to your proposal too. -- 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/
