Re: [PATCH 09/16] sched/fair: Let asymmetric cpu configurations balance at wake-up

Wanpeng Li
2016-05-25T06:57:05Z

2016-05-23 18:58 GMT+08:00 Morten Rasmussen <morten.rasmussen@arm.com>:
> Currently, SD_WAKE_AFFINE always takes priority over wakeup balancing if
> SD_BALANCE_WAKE is set on the sched_domains. For asymmetric
> configurations SD_WAKE_AFFINE is only desirable if the waking task's
> compute demand (utilization) is suitable for the cpu capacities
> available within the SD_WAKE_AFFINE sched_domain. If not, let wakeup
> balancing take over (find_idlest_{group, cpu}()).
>
> The assumption is that SD_WAKE_AFFINE is never set for a sched_domain
> containing cpus with different capacities. This is enforced by a
> previous patch based on the SD_ASYM_CPUCAPACITY flag.
>
> Ideally, we shouldn't set 'want_affine' in the first place, but we don't
> know if SD_BALANCE_WAKE is enabled on the sched_domain(s) until we start
> traversing them.
>
> cc: Ingo Molnar <mingo@redhat.com>
> cc: Peter Zijlstra <peterz@infradead.org>
>
> Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
> ---
> kernel/sched/fair.c | 28 +++++++++++++++++++++++++++-
> 1 file changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 564215d..ce44fa7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -114,6 +114,12 @@ unsigned int __read_mostly sysctl_sched_shares_window = 10000000UL;
> unsigned int sysctl_sched_cfs_bandwidth_slice = 5000UL;
> #endif
>
> +/*
> + * The margin used when comparing utilization with cpu capacity:
> + * util * 1024 < capacity * margin
> + */
> +unsigned int capacity_margin = 1280; /* ~20% */
> +
> static inline void update_load_add(struct load_weight *lw, unsigned long inc)
> {
> lw->weight += inc;
> @@ -5293,6 +5299,25 @@ static int cpu_util(int cpu)
> return (util >= capacity) ? capacity : util;
> }
>
> +static inline int task_util(struct task_struct *p)
> +{
> + return p->se.avg.util_avg;
> +}
> +
> +static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
> +{
> + long delta;
> + long prev_cap = capacity_of(prev_cpu);
> +
> + delta = cpu_rq(cpu)->rd->max_cpu_capacity - prev_cap;
> +
> + /* prev_cpu is fairly close to max, no need to abort wake_affine */
> + if (delta < prev_cap >> 3)
> + return 0;
> +
> + return prev_cap * 1024 < task_util(p) * capacity_margin;
> +}

If one task util_avg is SCHED_CAPACITY_SCALE and running on x86 box w/
SMT enabled, then each HT has capacity 589, wake_cap() will result in
always not wake affine, right?

Regards,
Wanpeng Li

> +
> /*
> * select_task_rq_fair: Select target runqueue for the waking task in domains
> * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE,
> @@ -5316,7 +5341,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
>
> if (sd_flag & SD_BALANCE_WAKE) {
> record_wakee(p);
> - want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
> + want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
> + && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
> }
>
> rcu_read_lock();
> --
> 1.9.1
>

--
Regards,
Wanpeng Li