Messages in this thread Patch in this message | | | Date | Fri, 21 Nov 2014 12:37:33 +0000 | From | Morten Rasmussen <> | Subject | Re: [PATCH v9 10/10] sched: move cfs task on a CPU with higher capacity |
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On Mon, Nov 03, 2014 at 04:54:47PM +0000, Vincent Guittot wrote: > When a CPU is used to handle a lot of IRQs or some RT tasks, the remaining > capacity for CFS tasks can be significantly reduced. Once we detect such > situation by comparing cpu_capacity_orig and cpu_capacity, we trig an idle > load balance to check if it's worth moving its tasks on an idle CPU. > > Once the idle load_balance has selected the busiest CPU, it will look for an > active load balance for only two cases : > - there is only 1 task on the busiest CPU. > - we haven't been able to move a task of the busiest rq. > > A CPU with a reduced capacity is included in the 1st case, and it's worth to > actively migrate its task if the idle CPU has got full capacity. This test has > been added in need_active_balance. > > As a sidenote, this will note generate more spurious ilb because we already > trig an ilb if there is more than 1 busy cpu. If this cpu is the only one that > has a task, we will trig the ilb once for migrating the task. > > The nohz_kick_needed function has been cleaned up a bit while adding the new > test > > env.src_cpu and env.src_rq must be set unconditionnally because they are used > in need_active_balance which is called even if busiest->nr_running equals 1 > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > kernel/sched/fair.c | 74 ++++++++++++++++++++++++++++++++++++++--------------- > 1 file changed, 53 insertions(+), 21 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index db392a6..02e8f7f 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6634,6 +6634,28 @@ static int need_active_balance(struct lb_env *env) > return 1; > } > > + /* > + * 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) ?
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.
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