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Message-Id: <9299CE62-C7B6-45F5-BD07-C1CB02F0D08C@joelfernandes.org>
Date: Tue, 29 Nov 2022 11:00:05 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: paulmck@...nel.org
Cc: "Zhang, Qiang1" <qiang1.zhang@...el.com>, frederic@...nel.org,
quic_neeraju@...cinc.com, rcu@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rcu-tasks: Make rude RCU-Tasks work well with CPU hotplug
> On Nov 29, 2022, at 10:18 AM, Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Tue, Nov 29, 2022 at 06:25:04AM +0000, Zhang, Qiang1 wrote:
>>>> On Nov 28, 2022, at 11:54 PM, Zhang, Qiang1 <qiang1.zhang@...el.com> wrote:
>>>
>>> On Mon, Nov 28, 2022 at 10:34:28PM +0800, Zqiang wrote:
>>>> Currently, invoke rcu_tasks_rude_wait_gp() to wait one rude
>>>> RCU-tasks grace period, if __num_online_cpus == 1, will return
>>>> directly, indicates the end of the rude RCU-task grace period.
>>>> suppose the system has two cpus, consider the following scenario:
>>>>
>>>> CPU0 CPU1 (going offline)
>>>> migration/1 task:
>>>> cpu_stopper_thread
>>>> -> take_cpu_down
>>>> -> _cpu_disable
>>>> (dec __num_online_cpus)
>>>> ->cpuhp_invoke_callback
>>>> preempt_disable
>>>> access old_data0
>>>> task1
>>>> del old_data0 .....
>>>> synchronize_rcu_tasks_rude()
>>>> task1 schedule out
>>>> ....
>>>> task2 schedule in
>>>> rcu_tasks_rude_wait_gp()
>>>> ->__num_online_cpus == 1
>>>> ->return
>>>> ....
>>>> task1 schedule in
>>>> ->free old_data0
>>>> preempt_enable
>>>>
>>>> when CPU1 dec __num_online_cpus and __num_online_cpus is equal one,
>>>> the CPU1 has not finished offline, stop_machine task(migration/1)
>>>> still running on CPU1, maybe still accessing 'old_data0', but the
>>>> 'old_data0' has freed on CPU0.
>>>>
>>>> This commit add cpus_read_lock/unlock() protection before accessing
>>>> __num_online_cpus variables, to ensure that the CPU in the offline
>>>> process has been completed offline.
>>>>
>>>> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
>>>>
>>>> First, good eyes and good catch!!!
>>>>
>>>> The purpose of that check for num_online_cpus() is not performance
>>>> on single-CPU systems, but rather correct operation during early boot.
>>>> So a simpler way to make that work is to check for RCU_SCHEDULER_RUNNING,
>>>> for example, as follows:
>>>>
>>>> if (rcu_scheduler_active != RCU_SCHEDULER_RUNNING &&
>>>> num_online_cpus() <= 1)
>>>> return; // Early boot fastpath for only one CPU.
>>>
>>> Hi Paul
>>>
>>> During system startup, because the RCU_SCHEDULER_RUNNING is set after starting other CPUs,
>>>
>>> CPU0 CPU1
>>>
>>> if (rcu_scheduler_active !=
>>> RCU_SCHEDULER_RUNNING &&
>>> __num_online_cpus == 1)
>>> return; inc __num_online_cpus
>>> (__num_online_cpus == 2)
>>>
>>> CPU0 didn't notice the update of the __num_online_cpus variable by CPU1 in time
>>> Can we move rcu_set_runtime_mode() before smp_init()
>>> any thoughts?
>>>
>>> Is anyone expected to do rcu-tasks operation before the scheduler is running?
>>
>> Not sure if such a scenario exists.
>>
>>> Typically this requires the tasks to context switch which is a scheduler operation.
>>>
>>> If the scheduler is not yet running, then I don’t think missing an update the __num_online_cpus matters since no one does a tasks-RCU synchronize.
>>
>> Hi Joel
>>
>> After the kernel_init task runs, before calling smp_init() to starting other CPUs,
>> the scheduler haven been initialization, task context switching can occur.
>
> Good catch, thank you both. For some reason, I was thinking that the
> additional CPUs did not come online until later.
>
> So how about this?
>
> if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
> return; // Early boot fastpath.
>
> If this condition is true, there is only one CPU and no scheduler,
> thus no preemption.
Agreed. I was going to suggest exactly this :)
Ack.
(Replying by phone but feel free to add my reviewed by tag).
- Joel
>
> Thanx, Paul
>
>> Thanks
>> Zqiang
>>
>>>
>>> Or did I miss something?
>>>
>>> Thanks.
>>>
>>>
>>>
>>>
>>> Thanks
>>> Zqiang
>>>
>>>>
>>>> This works because rcu_scheduler_active is set to RCU_SCHEDULER_RUNNING
>>>> long before it is possible to offline CPUs.
>>>>
>>>> Yes, schedule_on_each_cpu() does do cpus_read_lock(), again, good eyes,
>>>> and it also unnecessarily does the schedule_work_on() the current CPU,
>>>> but the code calling synchronize_rcu_tasks_rude() is on high-overhead
>>>> code paths, so this overhead is down in the noise.
>>>>
>>>> Until further notice, anyway.
>>>>
>>>> So simplicity is much more important than performance in this code.
>>>> So just adding the check for RCU_SCHEDULER_RUNNING should fix this,
>>>> unless I am missing something (always possible!).
>>>>
>>>> Thanx, Paul
>>>>
>>>> ---
>>>> kernel/rcu/tasks.h | 20 ++++++++++++++++++--
>>>> 1 file changed, 18 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
>>>> index 4a991311be9b..08e72c6462d8 100644
>>>> --- a/kernel/rcu/tasks.h
>>>> +++ b/kernel/rcu/tasks.h
>>>> @@ -1033,14 +1033,30 @@ static void rcu_tasks_be_rude(struct work_struct *work)
>>>> {
>>>> }
>>>>
>>>> +static DEFINE_PER_CPU(struct work_struct, rude_work);
>>>> +
>>>> // Wait for one rude RCU-tasks grace period.
>>>> static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
>>>> {
>>>> + int cpu;
>>>> + struct work_struct *work;
>>>> +
>>>> + cpus_read_lock();
>>>> if (num_online_cpus() <= 1)
>>>> - return; // Fastpath for only one CPU.
>>>> + goto end;// Fastpath for only one CPU.
>>>>
>>>> rtp->n_ipis += cpumask_weight(cpu_online_mask);
>>>> - schedule_on_each_cpu(rcu_tasks_be_rude);
>>>> + for_each_online_cpu(cpu) {
>>>> + work = per_cpu_ptr(&rude_work, cpu);
>>>> + INIT_WORK(work, rcu_tasks_be_rude);
>>>> + schedule_work_on(cpu, work);
>>>> + }
>>>> +
>>>> + for_each_online_cpu(cpu)
>>>> + flush_work(per_cpu_ptr(&rude_work, cpu));
>>>> +
>>>> +end:
>>>> + cpus_read_unlock();
>>>> }
>>>>
>>>> void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
>>>> --
>>>> 2.25.1
>>>>
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