Re: [PATCH v2] rcu-tasks: Make rude RCU-Tasks work well with CPU hotplug

From: Paul E. McKenney
Date: Tue Nov 29 2022 - 10:18:48 EST

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@xxxxxxxxx> 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@xxxxxxxxx>
> >>
> >> 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.

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
> >>