Re: [RFCv5 PATCH 41/46] sched/fair: add triggers for OPP change requests

Michael Turquette
2015-07-08T15:42:40Z

Hi Juri,

Quoting Morten Rasmussen (2015-07-07 11:24:24)
> From: Juri Lelli <juri.lelli@arm.com>
>
> Each time a task is {en,de}queued we might need to adapt the current
> frequency to the new usage. Add triggers on {en,de}queue_task_fair() for
> this purpose. Only trigger a freq request if we are effectively waking up
> or going to sleep. Filter out load balancing related calls to reduce the
> number of triggers.
>
> cc: Ingo Molnar <mingo@redhat.com>
> cc: Peter Zijlstra <peterz@infradead.org>
>
> Signed-off-by: Juri Lelli <juri.lelli@arm.com>
> ---
> kernel/sched/fair.c | 42 ++++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 40 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f74e9d2..b8627c6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4281,7 +4281,10 @@ static inline void hrtick_update(struct rq *rq)
> }
> #endif
>
> +static unsigned int capacity_margin = 1280; /* ~20% margin */

This is a 25% margin. Calling it ~20% is a bit misleading :)

Should margin be scaled for cpus that do not have max capacity == 1024?
In other words, should margin be dynamically calculated to be 20% of
*this* cpu's max capacity?

I'm imagining a corner case where a heterogeneous cpu system is set up
in such a way that adding margin that is hard-coded to 25% of 1024
almost always puts req_cap to the highest frequency, skipping some
reasonable capacity states in between.

> +
> static bool cpu_overutilized(int cpu);
> +static unsigned long get_cpu_usage(int cpu);
> struct static_key __sched_energy_freq __read_mostly = STATIC_KEY_INIT_FALSE;
>
> /*
> @@ -4332,6 +4335,26 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> if (!task_new && !rq->rd->overutilized &&
> cpu_overutilized(rq->cpu))
> rq->rd->overutilized = true;
> + /*
> + * We want to trigger a freq switch request only for tasks that
> + * are waking up; this is because we get here also during
> + * load balancing, but in these cases it seems wise to trigger
> + * as single request after load balancing is done.
> + *
> + * XXX: how about fork()? Do we need a special flag/something
> + * to tell if we are here after a fork() (wakeup_task_new)?
> + *
> + * Also, we add a margin (same ~20% used for the tipping point)
> + * to our request to provide some head room if p's utilization
> + * further increases.
> + */
> + if (sched_energy_freq() && !task_new) {
> + unsigned long req_cap = get_cpu_usage(cpu_of(rq));
> +
> + req_cap = req_cap * capacity_margin
> + >> SCHED_CAPACITY_SHIFT;

Probably a dumb question:

Can we "cheat" here and just assume that capacity and load use the same
units? That would avoid the multiplication and change your code to the
following:

#define capacity_margin SCHED_CAPACITY_SCALE >> 2; /* 25% */
req_cap += SCHED_CAPACITY_SCALE;

> + cpufreq_sched_set_cap(cpu_of(rq), req_cap);
> + }
> }
> hrtick_update(rq);
> }
> @@ -4393,6 +4416,23 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> if (!se) {
> sub_nr_running(rq, 1);
> update_rq_runnable_avg(rq, 1);
> + /*
> + * We want to trigger a freq switch request only for tasks that
> + * are going to sleep; this is because we get here also during
> + * load balancing, but in these cases it seems wise to trigger
> + * as single request after load balancing is done.
> + *
> + * Also, we add a margin (same ~20% used for the tipping point)
> + * to our request to provide some head room if p's utilization
> + * further increases.
> + */
> + if (sched_energy_freq() && task_sleep) {
> + unsigned long req_cap = get_cpu_usage(cpu_of(rq));
> +
> + req_cap = req_cap * capacity_margin
> + >> SCHED_CAPACITY_SHIFT;
> + cpufreq_sched_set_cap(cpu_of(rq), req_cap);

Filtering out the load_balance bits is neat.

Regards,
Mike

> + }
> }
> hrtick_update(rq);
> }
> @@ -4959,8 +4999,6 @@ static int find_new_capacity(struct energy_env *eenv,
> return idx;
> }
>
> -static unsigned int capacity_margin = 1280; /* ~20% margin */
> -
> static bool cpu_overutilized(int cpu)
> {
> return (capacity_of(cpu) * 1024) <
> --
> 1.9.1
>
> --
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