Messages in this thread | | | From | Vincent Guittot <> | Date | Tue, 8 Sep 2015 16:40:27 +0200 | Subject | Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity_orig |
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On 8 September 2015 at 16:35, Morten Rasmussen <morten.rasmussen@arm.com> wrote: > On Tue, Sep 08, 2015 at 04:06:36PM +0200, Vincent Guittot wrote: >> On 8 September 2015 at 14:52, Peter Zijlstra <peterz@infradead.org> wrote: >> > On Tue, Sep 08, 2015 at 02:26:06PM +0200, Peter Zijlstra wrote: >> >> On Tue, Sep 08, 2015 at 09:22:05AM +0200, Vincent Guittot wrote: >> >> > No, but >> >> > sa->util_avg = (sa->util_sum << SCHED_CAPACITY_SHIFT) / LOAD_AVG_MAX; >> >> > will fix the unit issue. >> >> >> >> Tricky that, LOAD_AVG_MAX very much relies on the unit being 1<<10. >> >> >> >> And where load_sum already gets a factor 1024 from the weight >> >> multiplication, util_sum does not get such a factor, and all the scaling >> >> we do on it loose bits. >> >> >> >> So at the moment we go compute the util_avg value, we need to inflate >> >> util_sum with an extra factor 1024 in order to make it work. >> >> >> >> And seeing that we do the shift up on sa->util_sum without consideration >> >> of overflow, would it not make sense to add that factor before the >> >> scaling and into the addition? >> >> >> >> Now, given all that, units are a complete mess here, and I'd not mind >> >> something like: >> >> >> >> #if (SCHED_LOAD_SHIFT - SCHED_LOAD_RESOLUTION) != SCHED_CAPACITY_SHIFT >> >> #error "something usefull" >> >> #endif >> >> >> >> somewhere near here. >> > >> > Something like teh below.. >> > >> > Another thing to ponder; the downside of scaled_delta_w is that its >> > fairly likely delta is small and you loose all bits, whereas the weight >> > is likely to be large can could loose a fwe bits without issue. >> > >> > That is, in fixed point scaling like this, you want to start with the >> > biggest numbers, not the smallest, otherwise you loose too much. >> > >> > The flip side is of course that now you can share a multiplcation. >> > >> > --- a/kernel/sched/fair.c >> > +++ b/kernel/sched/fair.c >> > @@ -682,7 +682,7 @@ void init_entity_runnable_average(struct >> > sa->load_avg = scale_load_down(se->load.weight); >> > sa->load_sum = sa->load_avg * LOAD_AVG_MAX; >> > sa->util_avg = scale_load_down(SCHED_LOAD_SCALE); >> > - sa->util_sum = LOAD_AVG_MAX; >> > + sa->util_sum = sa->util_avg * LOAD_AVG_MAX; >> > /* when this task enqueue'ed, it will contribute to its cfs_rq's load_avg */ >> > } >> > >> > @@ -2515,6 +2515,10 @@ static u32 __compute_runnable_contrib(u6 >> > return contrib + runnable_avg_yN_sum[n]; >> > } >> > >> > +#if (SCHED_LOAD_SHIFT - SCHED_LOAD_RESOLUTION) != 10 || SCHED_CAPACITY_SHIFT != 10 >> > +#error "load tracking assumes 2^10 as unit" >> > +#endif >> >> so why don't we set SCHED_CAPACITY_SHIFT to SCHED_LOAD_SHIFT ? > > Don't you mean: > > #define SCHED_LOAD_SHIFT (SCHED_CAPACITY_SHIFT + SCHED_LOAD_RESOLUTION)
yes you're right
> > ? > > Or do you want to increase the capacity resolution as well if you > increase the load resolution?
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