Messages in this thread | | | Date | Sat, 12 Feb 2022 19:26:08 +0530 | Subject | Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs | From | Mukesh Ojha <> |
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On 2/12/2022 4:58 PM, Neeraj Upadhyay wrote: > Hi Mukesh, > > On 2/12/2022 2:17 PM, Mukesh Ojha wrote: >> >> On 2/12/2022 3:44 AM, Paul E. McKenney wrote: >>> On Sat, Feb 12, 2022 at 12:14:20AM +0530, Mukesh Ojha wrote: >>>> On 2/10/2022 3:36 AM, Paul E. McKenney wrote: >>>>> On Wed, Feb 09, 2022 at 11:53:33PM +0530, Mukesh Ojha wrote: >>>>>> On 2/5/2022 4:25 AM, Paul E. McKenney wrote: >>>>>>> Although it is usually safe to invoke >>>>>>> synchronize_rcu_expedited() from a >>>>>>> preemption-enabled CPU-hotplug notifier, if it is invoked from a >>>>>>> notifier >>>>>>> between CPUHP_AP_RCUTREE_ONLINE and CPUHP_AP_ACTIVE, its >>>>>>> attempts to >>>>>>> invoke a workqueue handler will hang due to RCU waiting on a CPU >>>>>>> that >>>>>>> the scheduler is not paying attention to. This commit therefore >>>>>>> expands >>>>>>> use of the existing workqueue-independent >>>>>>> synchronize_rcu_expedited() >>>>>>> from early boot to also include CPUs that are being hotplugged. >>>>>>> >>>>>>> Link: >>>>>>> https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@quicinc.com/ >>>>>>> >>>>>>> Reported-by: Mukesh Ojha <quic_mojha@quicinc.com> >>>>>>> Cc: Tejun Heo <tj@kernel.org> >>>>>>> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> >>>>>>> --- >>>>>>> kernel/rcu/tree_exp.h | 14 ++++++++++---- >>>>>>> 1 file changed, 10 insertions(+), 4 deletions(-) >>>>>>> >>>>>>> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h >>>>>>> index 60197ea24ceb9..1a45667402260 100644 >>>>>>> --- a/kernel/rcu/tree_exp.h >>>>>>> +++ b/kernel/rcu/tree_exp.h >>>>>>> @@ -816,7 +816,7 @@ static int rcu_print_task_exp_stall(struct >>>>>>> rcu_node *rnp) >>>>>>> */ >>>>>>> void synchronize_rcu_expedited(void) >>>>>>> { >>>>>>> - bool boottime = (rcu_scheduler_active == RCU_SCHEDULER_INIT); >>>>>>> + bool no_wq; >>>>>>> struct rcu_exp_work rew; >>>>>>> struct rcu_node *rnp; >>>>>>> unsigned long s; >>>>>>> @@ -841,9 +841,15 @@ void synchronize_rcu_expedited(void) >>>>>>> if (exp_funnel_lock(s)) >>>>>>> return; /* Someone else did our work for us. */ >>>>>>> + /* Don't use workqueue during boot or from an incoming CPU. */ >>>>>>> + preempt_disable(); >>>>>>> + no_wq = rcu_scheduler_active == RCU_SCHEDULER_INIT || >>>>>>> + !cpumask_test_cpu(smp_processor_id(), cpu_active_mask); >>>>>>> + preempt_enable(); >>>>>>> + >>>>>>> /* Ensure that load happens before action based on it. */ >>>>>>> - if (unlikely(boottime)) { >>>>>>> - /* Direct call during scheduler init and >>>>>>> early_initcalls(). */ >>>>>>> + if (unlikely(no_wq)) { >>>>>>> + /* Direct call for scheduler init, early_initcall()s, >>>>>>> and incoming CPUs. */ >>>>>>> rcu_exp_sel_wait_wake(s); >>>>>>> } else { >>>>>>> /* Marshall arguments & schedule the expedited grace >>>>>>> period. */ >>>>>>> @@ -861,7 +867,7 @@ void synchronize_rcu_expedited(void) >>>>>>> /* Let the next expedited grace period start. */ >>>>>>> mutex_unlock(&rcu_state.exp_mutex); >>>>>>> - if (likely(!boottime)) >>>>>>> + if (likely(!no_wq)) >>>>>>> destroy_work_on_stack(&rew.rew_work); >>>>>>> } >>>>>>> EXPORT_SYMBOL_GPL(synchronize_rcu_expedited); >>>>>> Can we reach a condition after this change where no_wq = true and >>>>>> during >>>>>> rcu_stall report where exp_task = 0 list and exp_mask contain >>>>>> only this cpu >>>>>> ? >>>>> Hello, Mukesh, and thank you for looking this over! >>>>> >>>>> At first glance, I do not believe that this can happen because the >>>>> expedited grace-period machinery avoids waiting on the current CPU. >>>>> (See sync_rcu_exp_select_node_cpus(), both the raw_smp_processor_id() >>>>> early in the function and the get_cpu() later in the function.) >>>>> >>>>> But please let me know if I am missing something here. >>>>> >>>>> But suppose that we could in fact reach this condition. What bad >>>>> thing >>>>> would happen? Other than a resched_cpu() having been invoked several >>>>> times on a not-yet-online CPU, of course. ;-) >>>> >>>> I thought more about this, what if synchronize_rcu_expedited thread >>>> got >>>> schedule out and run on some other cpu >>>> and we clear out cpu on which it ran next from exp_mask. >>>> >>>> Queuing the work on same cpu ensures that it will always be right >>>> cpu to >>>> clear out. >>>> Do you think this can happen ? >>> Indeed it might. >>> >>> But if it did, the scheduler would invoke RCU's hook, which is named >>> rcu_note_context_switch(), and do so on the pre-switch CPU. There are >>> two implementations for this function, one for CONFIG_PREEMPT=y >>> and another for CONFIG_PREEMPT=n. Both look to me like they invoke >>> rcu_report_exp_rdp() when needed, one directly and the other via the >>> CONFIG_PREEMPT=n variant of rcu_qs(). >>> >>> Am I missing something? >>> >>> >> >> There is a issue we are facing where exp_mask is not getting cleared >> and rcu_stall report that >> the cpu we are waiting on sometime in idle and sometime executing >> some other task but >> it is not clearing itself from exp_mask from a very long time and in >> all the instances exp_task list is NULL. > > Can you please check whether [1] is present in your tree? > Thanks Neeraj. It is not there, will check the results with this patch.
-Mukesh
> > > Thanks > Neeraj > > [1] > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/kernel/rcu/tree_exp.h?h=v5.17-rc3&id=81f6d49cce2d2fe507e3fddcc4a6db021d9c2e7b >> >> expmask = 8, ==> cpu3 >> >> [80235.522440][T12441] rcu: INFO: rcu_preempt detected expedited >> stalls on CPUs/tasks: { 3-... } 9163622 jiffies s: 634705 root: 0x8/. >> [80235.534757][T12441] rcu: blocking rcu_node structures: >> [80235.540102][T12441] Task dump for CPU 3: >> [80235.540118][T12441] task:core_ctl state:D stack: 0 pid: >> 172 ppid: 2 flags:0x00000008 >> [80235.540150][T12441] Call trace: >> [80235.540178][T12441] __switch_to+0x2a8/0x3ac >> [80235.540207][T12441] rcu_state+0x11b0/0x1480 >> >> >> [80299.010105][T12441] rcu: INFO: rcu_preempt detected expedited >> stalls on CPUs/tasks: { 3-... } 9179494 jiffies s: 634705 root: 0x8/. >> [80299.022623][T12441] rcu: blocking rcu_node structures: >> [80299.027924][T12441] Task dump for CPU 3: >> [80299.027942][T12441] task:swapper/3 state:R running task >> stack: 0 pid: 0 ppid: 1 flags:0x00000008 >> [80299.027993][T12441] Call trace: >> [80299.028025][T12441] __switch_to+0x2a8/0x3ac >> [80299.028051][T12441] 0xffffffc010113eb4 >> >> >> As we were not seeing this earlier. >> Below is compile tested patch, can we do something like this ? >> >> ==========================================><==================================================== >> >> >> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h >> index 6453ac5..f0332e4 100644 >> --- a/kernel/rcu/tree_exp.h >> +++ b/kernel/rcu/tree_exp.h >> @@ -812,10 +812,12 @@ static int rcu_print_task_exp_stall(struct >> rcu_node *rnp) >> */ >> void synchronize_rcu_expedited(void) >> { >> - bool no_wq; >> + bool no_wq = (rcu_scheduler_active == RCU_SCHEDULER_INIT); >> + bool is_active; >> struct rcu_exp_work rew; >> struct rcu_node *rnp; >> unsigned long s; >> + int next_cpu; >> >> RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) || >> lock_is_held(&rcu_lock_map) || >> @@ -837,19 +839,28 @@ void synchronize_rcu_expedited(void) >> if (exp_funnel_lock(s)) >> return; /* Someone else did our work for us. */ >> >> - /* Don't use workqueue during boot or from an incoming CPU. */ >> - preempt_disable(); >> - no_wq = rcu_scheduler_active == RCU_SCHEDULER_INIT || >> - !cpumask_test_cpu(smp_processor_id(), cpu_active_mask); >> - preempt_enable(); >> - >> /* Ensure that load happens before action based on it. */ >> if (unlikely(no_wq)) { >> - /* Direct call during scheduler init, early_initcalls() and >> incoming CPUs. */ >> + /* Direct call during scheduler init, early_initcalls(). */ >> rcu_exp_sel_wait_wake(s); >> + mutex_unlock(&rcu_state.exp_mutex); >> + return; >> + } >> + >> + preempt_disable(); >> + is_active = cpumask_test_cpu(smp_processor_id(), cpu_active_mask); >> + preempt_enable(); >> + >> + rew.rew_s = s; >> + if (!is_active) { >> + INIT_WORK(&rew.rew_work, wait_rcu_exp_gp); >> + next_cpu = cpumask_next(smp_processor_id(), cpu_active_mask); >> + if (next_cpu >= nr_cpu_ids) >> + next_cpu = cpumask_first(cpu_active_mask); >> + >> + queue_work_on(next_cpu, rcu_gp_wq, &rew.rew_work); >> } else { >> /* Marshall arguments & schedule the expedited grace >> period. */ >> - rew.rew_s = s; >> INIT_WORK_ONSTACK(&rew.rew_work, wait_rcu_exp_gp); >> queue_work(rcu_gp_wq, &rew.rew_work); >> } >> @@ -863,7 +874,9 @@ void synchronize_rcu_expedited(void) >> /* Let the next expedited grace period start. */ >> mutex_unlock(&rcu_state.exp_mutex); >> >> - if (likely(!no_wq)) >> + if (likely(is_active)) >> destroy_work_on_stack(&rew.rew_work); >> + else >> + flush_work(&rew.rew_work); >> } >> EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
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