Re: [PATCH v2 00/14] Introducing TIF_NOTIFY_IPI flag

[Vincent Guittot <vincent.guittot@linaro.org>]

On Sat, 15 Jun 2024 at 03:28, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Jun 14, 2024 at 12:48:37PM +0200, Vincent Guittot wrote:
> > On Fri, 14 Jun 2024 at 11:28, Peter Zijlstra <peterz@infradead.org> wrote:
>
> > > > Vincent [5] pointed out a case where the idle load kick will fail to
> > > > run on an idle CPU since the IPI handler launching the ILB will check
> > > > for need_resched(). In such cases, the idle CPU relies on
> > > > newidle_balance() to pull tasks towards itself.
> > >
> > > Is this the need_resched() in _nohz_idle_balance() ? Should we change
> > > this to 'need_resched() && (rq->nr_running || rq->ttwu_pending)' or
> > > something long those lines?
> >
> > It's not only this but also in do_idle() as well which exits the loop
> > to look for tasks to schedule
>
> Is that really a problem? Reading the initial email the problem seems to
> be newidle balance, not hitting schedule. Schedule should be fairly
> quick if there's nothing to do, no?

There are 2 problems:
- Because of NEED_RESCHED being set, we go through the full schedule
path for no reason and we finally do a sched_balance_newidle()
- Because of need_resched being set o wake up the cpu, we will not
kick the softirq to run the nohz idle load balance and get a chance to
pull a task on an idle CPU

>
> > > I mean, it's fairly trivial to figure out if there really is going to be
> > > work there.
> > >
> > > > Using an alternate flag instead of NEED_RESCHED to indicate a pending
> > > > IPI was suggested as the correct approach to solve this problem on the
> > > > same thread.
> > >
> > > So adding per-arch changes for this seems like something we shouldn't
> > > unless there really is no other sane options.
> > >
> > > That is, I really think we should start with something like the below
> > > and then fix any fallout from that.
> >
> > The main problem is that need_resched becomes somewhat meaningless
> > because it doesn't  only mean "I need to resched a task" and we have
> > to add more tests around even for those not using polling
>
> True, however we already had some of that by having the wakeup list,
> that made nr_running less 'reliable'.
>
> The thing is, most architectures seem to have the TIF_POLLING_NRFLAG
> bit, even if their main idle routine isn't actually using it, much of

Yes, I'm surprised that Arm arch has the TIF_POLLING_NRFLAG whereas it
has never been supported by the arch

> the idle loop until it hits the arch idle will be having it set and will
> thus tickle these cases *sometimes*.
>
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 0935f9d4bb7b..cfa45338ae97 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -5799,7 +5800,7 @@ static inline struct task_struct *
> > >  __pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> > >  {
> > >         const struct sched_class *class;
> > > -       struct task_struct *p;
> > > +       struct task_struct *p = NULL;
> > >
> > >         /*
> > >          * Optimization: we know that if all tasks are in the fair class we can
> > > @@ -5810,9 +5811,11 @@ __pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> > >         if (likely(!sched_class_above(prev->sched_class, &fair_sched_class) &&
> > >                    rq->nr_running == rq->cfs.h_nr_running)) {
> > >
> > > -               p = pick_next_task_fair(rq, prev, rf);
> > > -               if (unlikely(p == RETRY_TASK))
> > > -                       goto restart;
> > > +               if (rq->nr_running) {
> >
> > How do you make the diff between a spurious need_resched() because of
> > polling and a cpu becoming idle ? isn't rq->nr_running null in both
> > cases ?
>
> Bah, true. It should also check current being idle, which then makes a
> mess of things again. Still, we shouldn't be calling newidle from idle,
> that's daft.
>
> I should probably not write code at 3am, but the below horror is what
> I came up with.
>
> ---
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0935f9d4bb7b..cfe8d3350819 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6343,19 +6344,12 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>   * Constants for the sched_mode argument of __schedule().
>   *
>   * The mode argument allows RT enabled kernels to differentiate a
> - * preemption from blocking on an 'sleeping' spin/rwlock. Note that
> - * SM_MASK_PREEMPT for !RT has all bits set, which allows the compiler to
> - * optimize the AND operation out and just check for zero.
> + * preemption from blocking on an 'sleeping' spin/rwlock.
>   */
> -#define SM_NONE                        0x0
> -#define SM_PREEMPT             0x1
> -#define SM_RTLOCK_WAIT         0x2
> -
> -#ifndef CONFIG_PREEMPT_RT
> -# define SM_MASK_PREEMPT       (~0U)
> -#else
> -# define SM_MASK_PREEMPT       SM_PREEMPT
> -#endif
> +#define SM_IDLE                        (-1)
> +#define SM_NONE                        0
> +#define SM_PREEMPT             1
> +#define SM_RTLOCK_WAIT         2
>
>  /*
>   * __schedule() is the main scheduler function.
> @@ -6396,11 +6390,12 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>   *
>   * WARNING: must be called with preemption disabled!
>   */
> -static void __sched notrace __schedule(unsigned int sched_mode)
> +static void __sched notrace __schedule(int sched_mode)
>  {
>         struct task_struct *prev, *next;
>         unsigned long *switch_count;
>         unsigned long prev_state;
> +       bool preempt = sched_mode > 0;
>         struct rq_flags rf;
>         struct rq *rq;
>         int cpu;
> @@ -6409,13 +6404,13 @@ static void __sched notrace __schedule(unsigned int sched_mode)
>         rq = cpu_rq(cpu);
>         prev = rq->curr;
>
> -       schedule_debug(prev, !!sched_mode);
> +       schedule_debug(prev, preempt);
>
>         if (sched_feat(HRTICK) || sched_feat(HRTICK_DL))
>                 hrtick_clear(rq);
>
>         local_irq_disable();
> -       rcu_note_context_switch(!!sched_mode);
> +       rcu_note_context_switch(preempt);
>
>         /*
>          * Make sure that signal_pending_state()->signal_pending() below
> @@ -6449,7 +6444,12 @@ static void __sched notrace __schedule(unsigned int sched_mode)
>          * that we form a control dependency vs deactivate_task() below.
>          */
>         prev_state = READ_ONCE(prev->__state);
> -       if (!(sched_mode & SM_MASK_PREEMPT) && prev_state) {
> +       if (sched_mode == SM_IDLE) {
> +               if (!rq->nr_running) {
> +                       next = prev;
> +                       goto picked;
> +               }
> +       } else if (!preempt && prev_state) {
>                 if (signal_pending_state(prev_state, prev)) {
>                         WRITE_ONCE(prev->__state, TASK_RUNNING);
>                 } else {
> @@ -6483,6 +6483,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
>         }
>
>         next = pick_next_task(rq, prev, &rf);
> +picked:
>         clear_tsk_need_resched(prev);
>         clear_preempt_need_resched();
>  #ifdef CONFIG_SCHED_DEBUG
> @@ -6521,9 +6522,9 @@ static void __sched notrace __schedule(unsigned int sched_mode)
>                 ++*switch_count;
>
>                 migrate_disable_switch(rq, prev);
>                 psi_sched_switch(prev, next, !task_on_rq_queued(prev));
>
> -               trace_sched_switch(sched_mode & SM_MASK_PREEMPT, prev, next, prev_state);
> +               trace_sched_switch(preempt, prev, next, prev_state);
>
>                 /* Also unlocks the rq: */
>                 rq = context_switch(rq, prev, next, &rf);
> @@ -6599,7 +6601,7 @@ static void sched_update_worker(struct task_struct *tsk)
>         }
>  }
>
> -static __always_inline void __schedule_loop(unsigned int sched_mode)
> +static __always_inline void __schedule_loop(int sched_mode)
>  {
>         do {
>                 preempt_disable();
> @@ -6644,7 +6646,7 @@ void __sched schedule_idle(void)
>          */
>         WARN_ON_ONCE(current->__state);
>         do {
> -               __schedule(SM_NONE);
> +               __schedule(SM_IDLE);
>         } while (need_resched());
>  }
>