Re: [PATCH v3] sched/core: introduce sched_core_idle_cpu()

[Joel Fernandes]

On Tue, Jul 4, 2023 at 1:40 AM Aaron Lu <aaron.lu@xxxxxxxxx> wrote:
>
> On Thu, Jun 29, 2023 at 12:02:04PM +0800, Cruz Zhao wrote:
> > As core scheduling introduced, a new state of idle is defined as
> > force idle, running idle task but nr_running greater than zero.
> >
> > If a cpu is in force idle state, idle_cpu() will return zero. This
> > result makes sense in some scenarios, e.g., load balance,
> > showacpu when dumping, and judge the RCU boost kthread is starving.
> >
> > But this will cause error in other scenarios, e.g., tick_irq_exit():
> > When force idle, rq->curr == rq->idle but rq->nr_running > 0, results
> > that idle_cpu() returns 0. In function tick_irq_exit(), if idle_cpu()
> > is 0, tick_nohz_irq_exit() will not be called, and ts->idle_active will
> > not become 1, which became 0 in tick_nohz_irq_enter().
> > ts->idle_sleeptime won't update in function update_ts_time_stats(), if
> > ts->idle_active is 0, which should be 1. And this bug will result that
> > ts->idle_sleeptime is less than the actual value, and finally will
> > result that the idle time in /proc/stat is less than the actual value.
> >
> > To solve this problem, we introduce sched_core_idle_cpu(), which
> > returns 1 when force idle. We audit all users of idle_cpu(), and
> > change idle_cpu() into sched_core_idle_cpu() in function
> > tick_irq_exit().
> >
> > v2-->v3: Only replace idle_cpu() with sched_core_idle_cpu() in
> > function tick_irq_exit(). And modify the corresponding commit log.
> >
> > Signed-off-by: Cruz Zhao <CruzZhao@xxxxxxxxxxxxxxxxx>
> > Reviewed-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > Reviewed-by: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> > Link: https://lore.kernel.org/lkml/1687631295-126383-1-git-send-email-CruzZhao@xxxxxxxxxxxxxxxxx
> > ---
> >  include/linux/sched.h |  2 ++
> >  kernel/sched/core.c   | 13 +++++++++++++
> >  kernel/softirq.c      |  2 +-
> >  3 files changed, 16 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index b09a83bfad8b..73e61c0f10a7 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -2430,9 +2430,11 @@ extern void sched_core_free(struct task_struct *tsk);
> >  extern void sched_core_fork(struct task_struct *p);
> >  extern int sched_core_share_pid(unsigned int cmd, pid_t pid, enum pid_type type,
> >                               unsigned long uaddr);
> > +extern int sched_core_idle_cpu(int cpu);
> >  #else
> >  static inline void sched_core_free(struct task_struct *tsk) { }
> >  static inline void sched_core_fork(struct task_struct *p) { }
> > +static inline int sched_core_idle_cpu(int cpu) { return idle_cpu(cpu); }
> >  #endif
> >
> >  extern void sched_set_stop_task(int cpu, struct task_struct *stop);
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 71c1a0f232b4..c80088956987 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -7286,6 +7286,19 @@ struct task_struct *idle_task(int cpu)
> >       return cpu_rq(cpu)->idle;
> >  }
> >
> > +#ifdef CONFIG_SCHED_CORE
> > +int sched_core_idle_cpu(int cpu)
> > +{
> > +     struct rq *rq = cpu_rq(cpu);
> > +
> > +     if (sched_core_enabled(rq) && rq->curr == rq->idle)
> > +             return 1;
>
> If the intention is to consider forced idle cpus as idle, then should
> the above condition written as:
>
>         if (sched_core_enabled(rq) && rq->core->core_forceidle_count)
>                 return 1;
> ?
>
> Or as long as a single cookied task is running, all normal idle cpus are
> regarded forced idle here and 1 is returned while previously, idle_cpu()
> is called for those cpus and if they have wakeup task pending, they are
> not regarded as idle so looks like a behaviour change.
>

Ah you're right, great insight. _sigh_ I should not have missed that
during review. It will change idle_cpu() behavior if core sched is
enabled...

 - Joel