On Thu, Jun 25, 2020 at 17:43:52 +0200, Qais Yousef <qais.you...@arm.com>
wrote...
[...]
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 235b2cae00a0..e2f1fffa013c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -794,6 +794,25 @@ unsigned int sysctl_sched_uclamp_util_max =
> SCHED_CAPACITY_SCALE;
> /* All clamps are required to be less or equal than these values */
> static struct uclamp_se uclamp_default[UCLAMP_CNT];
>
> +/*
> + * This static key is used to reduce the uclamp overhead in the fast path. It
> + * only disables the call to uclamp_rq_{inc, dec}() in
> enqueue/dequeue_task().
> + *
> + * This allows users to continue to enable uclamp in their kernel config with
> + * minimum uclamp overhead in the fast path.
> + *
> + * As soon as userspace modifies any of the uclamp knobs, the static key is
> + * enabled, since we have an actual users that make use of uclamp
> + * functionality.
> + *
> + * The knobs that would enable this static key are:
> + *
> + * * A task modifying its uclamp value with sched_setattr().
> + * * An admin modifying the sysctl_sched_uclamp_{min, max} via procfs.
> + * * An admin modifying the cgroup cpu.uclamp.{min, max}
> + */
> +static DEFINE_STATIC_KEY_FALSE(sched_uclamp_used);
This is me being choosy, but given that:
---8<---
% grep -e '_used[ )]' fair.c core.c
fair.c: return static_key_false(&__cfs_bandwidth_used);
fair.c: static_key_slow_inc_cpuslocked(&__cfs_bandwidth_used);
fair.c: static_key_slow_dec_cpuslocked(&__cfs_bandwidth_used);
% grep -e '_enabled[ )]' fair.c core.c
fair.c: if (!cfs_bandwidth_used() || !cfs_rq->runtime_enabled)
fair.c: if (!cfs_rq->runtime_enabled || cfs_rq->nr_running)
fair.c: if (!cfs_rq->runtime_enabled || cfs_rq->curr)
fair.c: if (likely(!cfs_rq->runtime_enabled || cfs_rq->runtime_remaining > 0))
fair.c: cfs_rq->runtime_enabled = 0;
fair.c: cfs_rq->runtime_enabled = cfs_b->quota != RUNTIME_INF;
fair.c: if (!cfs_rq->runtime_enabled)
fair.c: cfs_rq->runtime_enabled = 0;
core.c: if (static_key_false((¶virt_steal_rq_enabled))) {
core.c: if (unlikely(!p->sched_class->uclamp_enabled))
core.c: if (unlikely(!p->sched_class->uclamp_enabled))
core.c: * Prevent race between setting of cfs_rq->runtime_enabled and
core.c: runtime_enabled = quota != RUNTIME_INF;
core.c: runtime_was_enabled = cfs_b->quota != RUNTIME_INF;
core.c: if (runtime_enabled && !runtime_was_enabled)
core.c: if (runtime_enabled)
core.c: cfs_rq->runtime_enabled = runtime_enabled;
core.c: if (runtime_was_enabled && !runtime_enabled)
---8<---
even just for consistency shake, I would still prefer sched_uclamp_enabled for
the static key name.
> +
> /* Integer rounded range for each bucket */
> #define UCLAMP_BUCKET_DELTA DIV_ROUND_CLOSEST(SCHED_CAPACITY_SCALE,
> UCLAMP_BUCKETS)
>
> @@ -994,9 +1013,16 @@ static inline void uclamp_rq_dec_id(struct rq *rq,
> struct task_struct *p,
> lockdep_assert_held(&rq->lock);
>
> bucket = &uc_rq->bucket[uc_se->bucket_id];
> - SCHED_WARN_ON(!bucket->tasks);
> - if (likely(bucket->tasks))
> - bucket->tasks--;
> +
> + /*
> + * This could happen if sched_uclamp_used was enabled while the
> + * current task was running, hence we could end up with unbalanced call
> + * to uclamp_rq_dec_id().
> + */
> + if (unlikely(!bucket->tasks))
> + return;
> +
> + bucket->tasks--;
> uc_se->active = false;
In this chunk you are indeed changing the code.
Are we sure there are not issues with patterns like:
enqueue(taskA)
// uclamp gets enabled
enqueue(taskB)
dequeue(taskA)
// bucket->tasks is now 0
dequeue(taskB)
TaskB has been enqueued with with uclamp enabled, thus it
has got uc_se->active=True and enforced its clamp value at RQ level.
But with your change above we don't reset that anymore.
As per my previous proposal: why not just removing the SCHED_WARN_ON?
That's the only real problem in the code above, since now we are not
more granted to have balanced inc/dec.
[...]
> +bool uclamp_is_enabled(void)
> +{
> + return static_branch_likely(&sched_uclamp_used);
> +}
> +
The above and the following changes are not necessary if we just
add the guards at the beginning of uclamp_rq_util_with() instead of what
you do in PATCH1.
I think this will give better overheads removal by avoiding not
necessary min/max clamps for both RT and CFS.
> diff --git a/kernel/sched/cpufreq_schedutil.c
> b/kernel/sched/cpufreq_schedutil.c
> index 7fbaee24c824..3f4e296ccb67 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -210,7 +210,7 @@ unsigned long schedutil_cpu_util(int cpu, unsigned long
> util_cfs,
> unsigned long dl_util, util, irq;
> struct rq *rq = cpu_rq(cpu);
>
> - if (!IS_BUILTIN(CONFIG_UCLAMP_TASK) &&
> + if (!uclamp_is_enabled() &&
> type == FREQUENCY_UTIL && rt_rq_is_runnable(&rq->rt)) {
> return max;
> }
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 1d4e94c1e5fe..aff5430331e4 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2371,6 +2371,8 @@ unsigned long uclamp_rq_util_with(struct rq *rq,
> unsigned long util,
>
> return clamp(util, min_util, max_util);
> }
> +
> +bool uclamp_is_enabled(void);
> #else /* CONFIG_UCLAMP_TASK */
> static inline
> unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util,
> @@ -2378,6 +2380,11 @@ unsigned long uclamp_rq_util_with(struct rq *rq,
> unsigned long util,
> {
> return util;
> }
> +
> +static inline bool uclamp_is_enabled(void)
> +{
> + return false;
> +}
> #endif /* CONFIG_UCLAMP_TASK */
>
> #ifdef arch_scale_freq_capacity
