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

Morten Rasmussen
2016-05-25T09:49:11Z

On Wed, May 25, 2016 at 02:57:00PM +0800, Wanpeng Li wrote:
> 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?

The idea is that SMT systems would bail out already at the previous
condition. We should have max_cpu_capacity == prev_cap == 589, delta
should then be zero and make the first condition true and make
wake_cap() always return 0 for any system with symmetric capacities
regardless of their actual capacity values.

Note that this isn't entirely true as I used capacity_of() for prev_cap,
if I change that to capacity_orig_of() it should be true.

By making the !wake_cap() condition always true for want_affine, we
should preserve existing behaviour for SMT/SMP. The only overhead is the
capacity delta computation and comparison, which should be cheap.

Does that make sense?

Btw, task util_avg == SCHED_CAPACITY_SCALE should only be possible
temporarily, it should decay to util_avg <=
capacity_orig_of(task_cpu(p)) over time. That doesn't affect your
question though as the second condition would still evaluate true if
util_avg == capacity_orig_of(task_cpu(p)), but as said above the first
condition should bail out before we get here.

Morten

> > +
> > /*
> > * 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
> >