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Date: Tue, 29 Nov 2022 12:13:49 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Joel Fernandes <joel@...lfernandes.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 Tue, Nov 29, 2022 at 03:01:12PM -0500, Joel Fernandes wrote:
> > On Nov 29, 2022, at 2:18 PM, Paul E. McKenney <paulmck@...nel.org> wrote:
> > On Tue, Nov 29, 2022 at 11:00:05AM -0500, Joel Fernandes wrote:
> >>>> 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).
> >
> > I should add that the downside of this approach is that there is a short
> > time between the scheduler initializing and workqueues fully initializing
> > where a critical-path call to synchronize_rcu_tasks() will hang the
> > system. I do -not- consider this to be a real problem because RCU had
> > some hundreds of calls to synchronize_rcu() before this became an issue.
> >
> > So this should be fine, but please recall this for when/if someone does
> > stick a synchronize_rcu_tasks() into that short time. ;-)
>
> Thanks Paul, but why would anyone want to do sync rcu tasks, before
> the scheduler is fully initialized?
I could ask that same question of a number of other RCU API members. ;-)
> Maybe we can add a warning here in the if-early-return path, to make
> sure no such usage slips. And then we can look into someone using it
> that way, if they ever start using it.
I expect that it would be more work to code and maintain any such warning
than it would to diagnose the hang, so let's leave it as is.
Thanx, Paul
> Thanks,
>
> - Joel
>
> >
> > Thanx, Paul
> >
> >> - 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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