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SubjectRe: [PATCH 09/11] sched: use pelt for scale_rt_capacity()

* Vincent Guittot <vincent.guittot@linaro.org> wrote:

> The utilization of the CPU by rt, dl and interrupts are now tracked with
> PELT so we can use these metrics instead of rt_avg to evaluate the remaining
> capacity available for cfs class.
>
> scale_rt_capacity() behavior has been changed and now returns the remaining
> capacity available for cfs instead of a scaling factor because rt, dl and
> interrupt provide now absolute utilization value.
>
> The same formula as schedutil is used:
> irq util_avg + (1 - irq util_avg / max capacity ) * /Sum rq util_avg
> but the implementation is different because it doesn't return the same value
> and doesn't benefit of the same optimization
>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
> kernel/sched/deadline.c | 2 --
> kernel/sched/fair.c | 41 +++++++++++++++++++----------------------
> kernel/sched/pelt.c | 2 +-
> kernel/sched/rt.c | 2 --
> 4 files changed, 20 insertions(+), 27 deletions(-)

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d2758e3..ce0dcbf 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7550,39 +7550,36 @@ static inline int get_sd_load_idx(struct sched_domain *sd,
> static unsigned long scale_rt_capacity(int cpu)
> {
> struct rq *rq = cpu_rq(cpu);
> - u64 total, used, age_stamp, avg;
> - s64 delta;
> -
> - /*
> - * Since we're reading these variables without serialization make sure
> - * we read them once before doing sanity checks on them.
> - */
> - age_stamp = READ_ONCE(rq->age_stamp);
> - avg = READ_ONCE(rq->rt_avg);
> - delta = __rq_clock_broken(rq) - age_stamp;
> + unsigned long max = arch_scale_cpu_capacity(NULL, cpu);
> + unsigned long used, irq, free;
>
> - if (unlikely(delta < 0))
> - delta = 0;
> +#if defined(CONFIG_IRQ_TIME_ACCOUNTING) || defined(CONFIG_PARAVIRT_TIME_ACCOUNTING)
> + irq = READ_ONCE(rq->avg_irq.util_avg);
>
> - total = sched_avg_period() + delta;
> + if (unlikely(irq >= max))
> + return 1;
> +#endif

Note that 'irq' is unused outside that macro block, resulting in a new warning on
defconfig builds:

CC kernel/sched/fair.o
kernel/sched/fair.c: In function ‘scale_rt_capacity’:
kernel/sched/fair.c:7553:22: warning: unused variable ‘irq’ [-Wunused-variable]
unsigned long used, irq, free;
^~~

I have applied the delta fix below for simplicity, but what we really want is a
cleanup of that function to eliminate the #ifdefs. One solution would be to factor
out the 'irq' utilization value into a helper inline, and double check that if the
configs are off the compiler does the right thing and eliminates this identity
transformation for the irq==0 case:

free *= (max - irq);
free /= max;

If the compiler refuses to optimize this away (due to the zero and overflow
cases), try to find something more clever?

Thanks,

Ingo

kernel/sched/fair.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e3221db0511a..d5f7d521e448 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7550,7 +7550,10 @@ static unsigned long scale_rt_capacity(int cpu)
{
struct rq *rq = cpu_rq(cpu);
unsigned long max = arch_scale_cpu_capacity(NULL, cpu);
- unsigned long used, irq, free;
+ unsigned long used, free;
+#if defined(CONFIG_IRQ_TIME_ACCOUNTING) || defined(CONFIG_PARAVIRT_TIME_ACCOUNTING)
+ unsigned long irq;
+#endif

#if defined(CONFIG_IRQ_TIME_ACCOUNTING) || defined(CONFIG_PARAVIRT_TIME_ACCOUNTING)
irq = READ_ONCE(rq->avg_irq.util_avg);
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