On Wed, Jul 13, 2016 at 02:56:41PM +0200, Vincent Guittot wrote:
> On 22 June 2016 at 19:03, Morten Rasmussen <morten.rasmus...@arm.com> wrote:
> > 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 all the cpu capacities
> > available within the SD_WAKE_AFFINE sched_domain. If not, let wakeup
>
> instead of "suitable for all the cpu capacities available within the
> SD_WAKE_AFFINE sched_domain", should it be "suitable for local cpu and
> prev cpu" becasue you only check the capacity of these 2 CPUs.
Good point. I currently make the implicit assumption that capacity of local cpu
and prev cpu represent the capacity for all cpus their SD_WAKE_AFFINE
domains. It breaks if you should choose to have SD_WAKE_AFFINE on a
domain that spans both little and big cpus, as if local/prev cpu happens
to be big we assume that they are all big and let select_idle_sibling()
handle the task placement even for big tasks if local/prev cpu are both
big.
I don't see why anybody would want that kind of setup, but I think the
assumption should still be written down somewhere, either here or in a
comment in wake_cap() or both.
The next paragraph in the commit message mentions that we actually only
check waker cpu and prev_cpu capacity. Would it be more clear if we
extend that to something like:
This patch makes affine wake-ups conditional on whether both the waker
cpu and prev_cpu has sufficient capacity for the waking task, or
not, assuming that the cpu capacities within an SD_WAKE_AFFINE
domain are homogeneous.
Thoughts?
>
> Other than this comment for the commit message, the patch looks good to me
> Acked-by: Vincent Guittot <vincent.guit...@linaro.org>
Thanks,
Morten
>
> > balancing take over (find_idlest_{group, cpu}()).
> >
> > This patch makes affine wake-ups conditional on whether both the waker
> > cpu and prev_cpu has sufficient capacity for the waking task, or not.
> >
> > It is assumed that the sched_group(s) containing the waker cpu and
> > prev_cpu only contain cpu with the same capacity (homogeneous).
> >
> > 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 <mi...@redhat.com>
> > cc: Peter Zijlstra <pet...@infradead.org>
> >
> > Signed-off-by: Morten Rasmussen <morten.rasmus...@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 216db302e87d..dba02c7b57b3 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;
> > @@ -5260,6 +5266,25 @@ 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 min_cap, max_cap;
> > +
> > + min_cap = min(capacity_orig_of(prev_cpu), capacity_orig_of(cpu));
> > + max_cap = cpu_rq(cpu)->rd->max_cpu_capacity;
> > +
> > + /* Minimum capacity is close to max, no need to abort wake_affine */
> > + if (max_cap - min_cap < max_cap >> 3)
> > + return 0;
> > +
> > + return min_cap * 1024 < task_util(p) * capacity_margin;
> > +}
> > +
> > /*
> > * 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,
> > @@ -5283,7 +5308,8 @@ 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
> >
