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

From: Vincent Guittot
Date: Fri Jun 14 2024 - 06:49:28 EST

On Fri, 14 Jun 2024 at 11:28, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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

> + 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) {