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

Chen Yu
2024-06-14T16:31:58Z

On 2024-06-14 at 12:48:37 +0200, Vincent Guittot wrote:
> On Fri, 14 Jun 2024 at 11:28, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Thu, Jun 13, 2024 at 06:15:59PM +0000, K Prateek Nayak wrote:
> > > Effects of call_function_single_prep_ipi()
> > > ==========================================
> > >
> > > To pull a TIF_POLLING thread out of idle to process an IPI, the sender
> > > sets the TIF_NEED_RESCHED bit in the idle task's thread info in
> > > call_function_single_prep_ipi() and avoids sending an actual IPI to the
> > > target. As a result, the scheduler expects a task to be enqueued when
> > > exiting the idle path. This is not the case with non-polling idle states
> > > where the idle CPU exits the non-polling idle state to process the
> > > interrupt, and since need_resched() returns false, soon goes back to
> > > idle again.
> > >
> > > When TIF_NEED_RESCHED flag is set, do_idle() will call schedule_idle(),
> > > a large part of which runs with local IRQ disabled. In case of ipistorm,
> > > when measuring IPI throughput, this large IRQ disabled section delays
> > > processing of IPIs. Further auditing revealed that in absence of any
> > > runnable tasks, pick_next_task_fair(), which is called from the
> > > pick_next_task() fast path, will always call newidle_balance() in this
> > > scenario, further increasing the time spent in the IRQ disabled section.
> > >
> > > Following is the crude visualization of the problem with relevant
> > > functions expanded:
> > > --
> > > CPU0 CPU1
> > > ==== ====
> > > do_idle() {
> > > __current_set_polling();
> > > ...
> > > monitor(addr);
> > > if (!need_resched())
> > > mwait() {
> > > /* Waiting */
> > > smp_call_function_single(CPU1, func, wait = 1) { ...
> > > ... ...
> > > set_nr_if_polling(CPU1) { ...
> > > /* Realizes CPU1 is polling */ ...
> > > try_cmpxchg(addr, ...
> > > &val, ...
> > > val | _TIF_NEED_RESCHED); ...
> > > } /* Does not send an IPI */ ...
> > > ... } /* mwait exit due to write at addr */
> > > csd_lock_wait() { }
> > > /* Waiting */ preempt_set_need_resched();
> > > ... __current_clr_polling();
> > > ... flush_smp_call_function_queue() {
> > > ... func();
> > > } /* End of wait */ }
> > > } schedule_idle() {
> > > ...
> > > local_irq_disable();
> > > smp_call_function_single(CPU1, func, wait = 1) { ...
> > > ... ...
> > > arch_send_call_function_single_ipi(CPU1); ...
> > > \ ...
> > > \ newidle_balance() {
> > > \ ...
> > > /* Delay */ ...
> > > \ }
> > > \ ...
> > > \--------------> local_irq_enable();
> > > /* Processes the IPI */
> > > --
> > >
> > >
> > > Skipping newidle_balance()
> > > ==========================
> > >
> > > In an earlier attempt to solve the challenge of the long IRQ disabled
> > > section, newidle_balance() was skipped when a CPU waking up from idle
> > > was found to have no runnable tasks, and was transitioning back to
> > > idle [2]. Tim [3] and David [4] had pointed out that newidle_balance()
> > > may be viable for CPUs that are idling with tick enabled, where the
> > > newidle_balance() has the opportunity to pull tasks onto the idle CPU.
> >
> > I don't think we should be relying on this in any way shape or form.
> > NOHZ can kill that tick at any time.
> >
> > Also, semantically, calling newidle from the idle thread is just daft.
> > You're really not newly idle in that case.
> >
> > > 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
>
> >
> > 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
>
> >
> > 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 ?
> In the later case, we need to call sched_balance_newidle() but not in the former
>

Not sure if I understand correctly, if the goal of smp_call_function_single() is to
kick the idle CPU and do not force it to launch the schedule()->sched_balance_newidle(),
can we set the _TIF_POLLING_NRFLAG rather than _TIF_NEED_RESCHED in set_nr_if_polling()?
I think writing any value to the monitor address would wakeup the idle CPU. And _TIF_POLLING_NRFLAG
will be cleared once that idle CPU exit the idle loop, so we don't introduce arch-wide flag.

thanks,
Chenyu

> > + p = pick_next_task_fair(rq, prev, rf);
> > + if (unlikely(p == RETRY_TASK))
> > + goto restart;
> > + }
> >
> > /* Assume the next prioritized class is idle_sched_class */
> > if (!p) {