On Thu, Nov 30, 2017 at 11:47:18AM +0000, Patrick Bellasi wrote:
> Currently, sg_cpu's flags are set to the value defined by the last call
> of the cpufreq_update_util(); for RT/DL classes this corresponds to the
> SCHED_CPUFREQ_{RT/DL} flags always being set.
>
> When multiple CPUs share the same frequency domain it might happen that
> a CPU which executed an RT task, right before entering IDLE, has one of
> the SCHED_CPUFREQ_RT_DL flags set, permanently, until it exits IDLE.
>
> Although such an idle CPU is _going to be_ ignored by the
> sugov_next_freq_shared():
> 1. this kind of "useless RT requests" are ignored only if more then
> TICK_NSEC have elapsed since the last update
> 2. we can still potentially trigger an already too late switch to
> MAX, which starts also a new throttling interval
> 3. the internal state machine is not consistent with what the
> scheduler knows, i.e. the CPU is now actually idle
So I _really_ hate having to clutter the idle path for this shared case
:/
1, can obviously be fixed by short-circuiting the timeout when idle.
2. not sure how if you do 1; anybody doing a switch will go through
sugov_next_freq_shared() which will poll all relevant CPUs and per 1
will see its idle, no?
Not sure what that leaves for 3.
> diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
> index d518664cce4f..6e8ae2aa7a13 100644
> --- a/kernel/sched/idle_task.c
> +++ b/kernel/sched/idle_task.c
> @@ -30,6 +30,10 @@ pick_next_task_idle(struct rq *rq, struct task_struct
> *prev, struct rq_flags *rf
> put_prev_task(rq, prev);
> update_idle_core(rq);
> schedstat_inc(rq->sched_goidle);
> +
> + /* kick cpufreq (see the comment in kernel/sched/sched.h). */
> + cpufreq_update_util(rq, SCHED_CPUFREQ_IDLE);
> +
> return rq->idle;
> }
>
> --
> 2.14.1
>
