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

Hello Chenyu,

On 6/14/2024 10:01 PM, Chen Yu wrote:
> On 2024-06-14 at 12:48:37 +0200, Vincent Guittot wrote:
>> On Fri, 14 Jun 2024 at 11:28, Peter Zijlstra <[email protected]> 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.
Although this might work for MWAIT, there is no way for the generic idle
path to know if there is a pending interrupt within a TIF_POLLING_NRFLAG
section. do_idle() sets TIF_POLLING_NRFLAG and relies on a bunch of
need_resched() checks along the way to bail early until finally doing a
current_clr_polling_and_test() before handing off to the cpuidle driver
in call_cpuidle(). I believe this section will necessarily need the sender
to indicate a pending interrupt via TIF_NEED_RESCHED flag to enable the
early bail out before going into the cpuidle driver since this case cannot
be considered the same as a break from MWAIT.

On x86, there seems to be a possibility of missing an interrupt if
someone writes _TIF_POLLING_NRFLAG to thread info between the target
executing MONTOR and MWAIT. AMD64 Architecture Programmer’s Manual
Volume 3: "General-Purpose and System Instructions", Chapter 4. "System
Instruction Reference", section "MWAIT" carries the following note in
the coding requirements:

"MWAIT must be conditionally executed only if the awaited store has not
already occurred. (This prevents a race condition between the MONITOR
instruction arming the monitoring hardware and the store intended to
trigger the monitoring hardware.)"

There exists a similar note in the "Example" section for "MWAIT" in
Intel 64 and IA-32 Architectures Software Developer’s Manual, Vol 2B
Chapter 4.3 "Instructions (M-U)"

I'm not sure if one can use use _TIF_POLLING_NRFLAG alone and cover
all the cases but there might be some clever trick to make it all
work :)

> 
> 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) {

-- 
Thanks and Regards,
Prateek