Date:   Fri, 19 Aug 2022 04:42:29 +0800
From:   Schspa Shi <schspa@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     rostedt@...dmis.org, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org
Subject: Re: [RFC PATCH] irq_work: wakeup irq_workd when queued first
 rt_lazy work


Schspa Shi <schspa@...il.com> writes:

> Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:
>
>> On 2022-07-12 01:23:15 [+0800], Schspa Shi wrote:
>>> I want to know if this difference is by design.
>>
>> Yes. type1 (LAZY) does not need immediate action but can't be scheduled
>> regularly like a workqueue.
>>
>>> If this is by design, we have a problem that the irq_work of type2
>>> will not execute as quickly as expected, it may be delayed by the
>>> irq_work of type1.
>>> 
>>> Please consider the following scenarios:
>>> 
>>> If the CPU queued a type1 irq_work A, and then a type2 irq_work B.
>>> But we won't make B executed quickly, because we won't issue the IPI
>>> interrupt to wakeup irq_workd (the llist_add call will return false).
>>
>> But those two are different lists. So adding type1 to list1 does not
>> affect type2 with list2
>>
>
> No, this will be added to same list (lazy_list).
> All irq work without IRQ_WORK_HARD_IRQ flags will be added to lazy_list.
> Maybe my description of type2 is not clear, type2 irq work means neither
> the IRQ_WORK_LAZY flag nor the IRQ_WORK_HARD_IRQ flag is set.
>
>>> This PATCH will issue the IPI_IRQ_WORK to make B execute quickly.
>>> 
>>> One thing that needs to be optimized is that we now have
>>> lazy_list.node.llist and lazy_work_raised which need to be granted
>>> to be atomicity, disabled the local CPU IRQ to make this atomic.
>>> There should be a better way to make these two variants to be atomically
>>> and I can go in deep if this little problem is not by design, and need
>>> to be fixed.
>>> 
>>> If these two types of irq_work should be the same with the priority.
>>> maybe we should change.
>>> 
>>> if (!lazy_work || tick_nohz_tick_stopped()) {
>>> 	arch_irq_work_raise();
>>> }
>>> 
>>> to
>>> 
>>> if (!(lazy_work || rt_lazy_work) || tick_nohz_tick_stopped()) {
>>> 	arch_irq_work_raise();
>>> }
>>
>> but we wait for the timer for the lazy-work. RT has more LAZY items
>> compared to !RT. So if there is an error then it should be visible
>> there, too.
>>
>
> As type 2 work and type 1 work will be added to lazy_list, type 2 work
> can be delayed and have same priority as type 1.
>
>> Is there a problem with this? Adding (as you call it) type1 item does
>> not affect type2 items. They will will processed asap.
>>
>
> I noticed this because there is a BUG before the patch
> b4c6f86ec2f6 ("irq_work: Handle some irq_work in a per-CPU thread on PREEMPT_RT")
> applied, which makes the task hang on when the CPU hotplug.
>
> On some RT branches, lazy_work will be queued to ksoftirq via commit
> https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/kernel/irq_work.c?h=linux-5.10.y-rt&id=c1ecdc62c514c2d541490026c312ec614ebd35aa
> c1ecdc62c5 ("irqwork: push most work into softirq context")
>
> Which makes the irq_work won't be executed due to we don't call arch_irq_work_raise();
> and raise_softirq(TIMER_SOFTIRQ); won't be executed by this case too.
>
> If there is no timer exists on the current CPU, it will hang forever.
>
> Log as fellowing.
>
> [32987.846092] INFO: task core_ctl:749 blocked for more than 120 seconds.
> [32987.846106]       Tainted: G           O      5.10.59-rt52-g19228cd9c280-dirty #24
> [32987.846117] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [32987.846125] task:core_ctl        state:D stack:    0 pid:  749 ppid:     2 flags:0x00000028
> [32987.846149] Call trace:
> [32987.846155]  __switch_to+0x164/0x17c
> [32987.846175]  __schedule+0x4cc/0x5c0
> [32987.846190]  schedule+0x7c/0xcc
> [32987.846205]  schedule_timeout+0x34/0xdc
> [32987.846338]  do_wait_for_common+0xa0/0x12c
> [32987.846360]  wait_for_common+0x44/0x68
> [32987.846376]  wait_for_completion+0x18/0x24
> [32987.846391]  __cpuhp_kick_ap+0x58/0x68
> [32987.846408]  cpuhp_kick_ap+0x38/0x94
> [32987.846423]  _cpu_down+0xbc/0x1f8
> [32987.846443]  cpu_down_maps_locked+0x20/0x34
> [32987.846461]  cpu_device_down+0x24/0x40
> [32987.846477]  cpu_subsys_offline+0x10/0x1c
> [32987.846496]  device_offline+0x6c/0xbc
> [32987.846514]  remove_cpu+0x24/0x40
> [32987.846530]  do_core_ctl+0x44/0x88 [cpuhp_qos]
> [32987.846563]  try_core_ctl+0x90/0xb0 [cpuhp_qos]
> [32987.846587]  kthread+0x114/0x124
> [32987.846604]  ret_from_fork+0x10/0x30
>

Add missing logs to make this hang issues clear.

[32987.953814] NMI backtrace for cpu 3
[32987.953829] CPU: 3 PID: 30 Comm: cpuhp/3 Tainted: G           O      5.10.59-rt52-g19228cd9c280-dirty #24
[32987.953849] Hardware name: Horizon Robotics Journey 5 DVB (DT)
[32987.953859] pstate: 60c00009 (nZCv daif +PAN +UAO -TCO BTYPE=--)
[32987.953876] pc : irq_work_sync+0x8/0x1c
[32987.953900] lr : cpufreq_dbs_governor_stop+0x50/0x7c
[32987.953923] sp : ffff80001280bd20
[32987.953930] pmr_save: 000000e0
[32987.953937] x29: ffff80001280bd20 x28: 0000000000000000 
[32987.953960] x27: 0000000000000000 x26: 0000000000000000 
[32987.953980] x25: 0000000000000000 x24: 0000000000000001 
[32987.953999] x23: ffff800011403000 x22: ffff000183bf1800 
[32987.954019] x21: ffff80001155e8e0 x20: 0000000000000080 
[32987.954040] x19: ffff00018aafd400 x18: 0000000000000000 
[32987.954059] x17: 0000000000000000 x16: 0000000000000000 
[32987.954079] x15: 000000000000000a x14: 000000000000001e 
[32987.954098] x13: ffff800012e2703c x12: ffffffffffffffff 
[32987.954118] x11: ffffffffffffffff x10: 0000000000000a20 
[32987.954138] x9 : ffff80001280bab0 x8 : ffff000180274480 
[32987.954158] x7 : 00000000ffffffff x6 : ffff800012e255da 
[32987.954177] x5 : ffff80001280bd00 x4 : 0000000000000000 
[32987.954196] x3 : ffff80001280bd00 x2 : 0000000000000000 
[32987.954216] x1 : 0000000000000023 x0 : ffff00018aafd448 
[32987.954235] Call trace:
[32987.954243]  irq_work_sync+0x8/0x1c
[32987.954263]  cpufreq_stop_governor+0x6c/0x80
[32987.954282]  cpufreq_offline+0xc4/0x1ec
[32987.954300]  cpuhp_cpufreq_offline+0x10/0x20
[32987.954317]  cpuhp_invoke_callback+0xc0/0x1b0
[32987.954334]  cpuhp_thread_fun+0x124/0x170
[32987.954350]  smpboot_thread_fn+0x1e8/0x1ec
[32987.954367]  kthread+0x114/0x124
[32987.954384]  ret_from_fork+0x10/0x30

>
> Please notice this patch is only used to explain the problem, don't
> try to compile it.
>
>> Sebastian


-- 
BRs
Schspa Shi

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