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
>
On 20-Dec 15:33, Peter Zijlstra wrote:
> 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
> :/
:)
We would like to have per-CPU frequency domains... but the HW guys
always complain that's too costly from an HW/power standpoint...
and they are likely right :-/
So, here are are just at trying hard to have a SW status matching
the HW status... which is just another pain :-/
> 1, can obviously be fixed by short-circuiting the timeout when idle.
Mmm.. right... it should be possible for schedutil to detect that a
certain CPU is currently idle.
Can we use core.c::idle_cpu() from cpufreq_schedutil?
> 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?
Right, that should work...
> Not sure what that leaves for 3.
When a CPU is detected idle, perhaps we can still clear the RT flags...
... just for "consistency" of current status representation.
>
> > 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
> >
--
#include <best/regards.h>
Patrick Bellasi