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.