Messages in this thread | | | Date | Wed, 6 Sep 2023 16:38:47 +0200 | Subject | Re: [RFC PATCH 3/7] sched/fair: Remove magic margin in fits_capacity() | From | Dietmar Eggemann <> |
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On 28/08/2023 01:31, Qais Yousef wrote: > 80% margin is a magic value that has served its purpose for now, but it > no longer fits the variety of systems exist today. If a system is over > powered specifically, this 80% will mean we leave a lot of capacity > unused before we decide to upmigrate on HMP system. > > The upmigration behavior should rely on the fact that a bad decision > made will need load balance to kick in to perform misfit migration. And > I think this is an adequate definition for what to consider as enough > headroom to consider whether a util fits capacity or not. > > Use the new approximate_util_avg() function to predict the util if the > task continues to run for TICK_US. If the value is not strictly less > than the capacity, then it must not be placed there, ie considered > misfit. > > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io> > --- > kernel/sched/fair.c | 21 ++++++++++++++++++--- > 1 file changed, 18 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 0b7445cd5af9..facbf3eb7141 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -109,16 +109,31 @@ int __weak arch_asym_cpu_priority(int cpu) > } > > /* > - * The margin used when comparing utilization with CPU capacity. > + * The util will fit the capacity if it has enough headroom to grow within the > + * next tick - which is when any load balancing activity happens to do the > + * correction. > * > - * (default: ~20%) > + * If util stays within the capacity before tick has elapsed, then it should be > + * fine. If not, then a correction action must happen shortly after it starts > + * running, hence we treat it as !fit. > + * > + * TODO: TICK is not actually accurate enough. balance_interval is the correct > + * one to use as the next load balance doesn't not happen religiously at tick. > + * Accessing balance_interval might be tricky and will require some refactoring > + * first. > */
I understand that you want to have a more intelligent margin (depending on the util value) but why you want to use the time value of TICK_USEC (or the balance_interval)?
We call fits_capacity() e.g. in wakeup and the next lb can just happen immediately after it.
> -#define fits_capacity(cap, max) ((cap) * 1280 < (max) * 1024) > +static inline bool fits_capacity(unsigned long util, unsigned long capacity) > +{ > + return approximate_util_avg(util, TICK_USEC) < capacity; > +}
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