On Thu, 11 Mar 2021 at 13:05, Valentin Schneider <[email protected]> wrote: > > During load-balance, groups classified as group_misfit_task are filtered > out if they do not pass > > group_smaller_max_cpu_capacity(<candidate group>, <local group>); > > which itself employs fits_capacity() to compare the sgc->max_capacity of > both groups. > > Due to the underlying margin, fits_capacity(X, 1024) will return false for > any X > 819. Tough luck, the capacity_orig's on e.g. the Pixel 4 are > {261, 871, 1024}. If a CPU-bound task ends up on one of those "medium" > CPUs, misfit migration will never intentionally upmigrate it to a CPU of > higher capacity due to the aforementioned margin. > > One may argue the 20% margin of fits_capacity() is excessive in the advent > of counter-enhanced load tracking (APERF/MPERF, AMUs), but one point here > is that fits_capacity() is meant to compare a utilization value to a > capacity value, whereas here it is being used to compare two capacity > values. As CPU capacity and task utilization have different dynamics, a > sensible approach here would be to add a new helper dedicated to comparing > CPU capacities. > > Reviewed-by: Qais Yousef <[email protected]> > Signed-off-by: Valentin Schneider <[email protected]> > --- > kernel/sched/fair.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index db892f6e222f..ddb2ab3edf6d 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -113,6 +113,13 @@ int __weak arch_asym_cpu_priority(int cpu) > */ > #define fits_capacity(cap, max) ((cap) * 1280 < (max) * 1024) > > +/* > + * The margin used when comparing CPU capacities. > + * is 'cap1' noticeably greater than 'cap2' > + * > + * (default: ~5%) > + */ > +#define capacity_greater(cap1, cap2) ((cap1) * 1024 > (cap2) * 1078)
defined but not used. Should be merged with next patch which start to use it > #endif > > #ifdef CONFIG_CFS_BANDWIDTH > -- > 2.25.1 >
