On Mon, Nov 24, 2014 at 02:45:45PM +0000, Vincent Guittot wrote: > On 21 November 2014 at 13:37, Morten Rasmussen <[email protected]> > wrote: > > On Mon, Nov 03, 2014 at 04:54:47PM +0000, Vincent Guittot wrote: > > >> > >> + /* > >> + * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task. > >> + * It's worth migrating the task if the src_cpu's capacity is reduced > >> + * because of other sched_class or IRQs whereas capacity stays > >> + * available on dst_cpu. > >> + */ > >> + if ((env->idle != CPU_NOT_IDLE) && > >> + (env->src_rq->cfs.h_nr_running == 1)) { > >> + unsigned long src_eff_capacity, dst_eff_capacity; > >> + > >> + dst_eff_capacity = 100; > >> + dst_eff_capacity *= capacity_of(env->dst_cpu); > >> + dst_eff_capacity *= capacity_orig_of(env->src_cpu); > >> + > >> + src_eff_capacity = sd->imbalance_pct; > >> + src_eff_capacity *= capacity_of(env->src_cpu); > >> + src_eff_capacity *= capacity_orig_of(env->dst_cpu); > > > > Do we need to scale by capacity_orig? Shouldn't the absolute capacity be > > better? > > > > if (capacity_of(env->src) * sd->imbalance_pct < capacity_of(env->dst) * > > 100) ? > > we don't want to compare absolute capacity between CPUs but to compare > the reduction of their capacity because we want to choose the CPU > which is less used by RT tasks or irq
But least relative RT load doesn't necessarily mean most available compute capacity. 50% RT use of a capacity_orig = 1000 (capacity_of(cpu) = 500, eff_capacity = 50%) gives better CFS throughput than 20% RT use of a capacity_orig = 500 (capacity_of(cpu) = 400, eff_capacity = 80%). Why pick the cpu with less throughput? Morten > > Regards, > Vincent > > > > Isn't it the absolute available capacity that matters? For SMP > > capacity_orig is the same and cancels out and doesn't change anything. > > For big.LITTLE we would rather have the task run on a big where rt/irq > > eats 30% than a little cpu where rq/irq eats 5%, assuming big capacity > > is much bigger than little capacity so the absolute available capacity > > (~cycles/time) is larger on the big cpu. > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to [email protected] > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
