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Subject: Re: [PATCH RESEND] mm: fix tick_sched timer blocked by
 pgdat_resize_lock
To:     Kirill Tkhai <ktkhai@virtuozzo.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@soleen.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20200110082510.172517-2-shile.zhang@linux.alibaba.com>
 <20200110093053.34777-1-shile.zhang@linux.alibaba.com>
 <1ee6088c-9e72-8824-3a9a-fc099d196faf@virtuozzo.com>
 <c7ac0338-78a6-2ae3-465c-2d6371d96a72@linux.alibaba.com>
 <9420eab3-5e5e-150f-53c9-6cd40bacf859@virtuozzo.com>
From:   Shile Zhang <shile.zhang@linux.alibaba.com>
Message-ID: <ba242ee6-22be-3047-5a88-e6b39e1509ef@linux.alibaba.com>
Date:   Tue, 14 Jan 2020 16:54:22 +0800
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On 2020/1/13 16:11, Kirill Tkhai wrote:
> On 13.01.2020 03:54, Shile Zhang wrote:
>>
>> On 2020/1/10 19:42, Kirill Tkhai wrote:
>>> On 10.01.2020 12:30, Shile Zhang wrote:
>>>> When 'CONFIG_DEFERRED_STRUCT_PAGE_INIT' is set, 'pgdat_resize_lock'
>>>> will be called inside 'pgdatinit' kthread to initialise the deferred
>>>> pages with local interrupts disabled. Which is introduced by
>>>> commit 3a2d7fa8a3d5 ("mm: disable interrupts while initializing deferred
>>>> pages").
>>>>
>>>> But 'pgdatinit' kthread is possible be pined on the boot CPU (CPU#0 by
>>>> default), especially in small system with NRCPUS <= 2. In this case, the
>>>> interrupts are disabled on boot CPU during memory initialising, which
>>>> caused the tick_sched timer be blocked, leading to wall clock stuck.
>>>>
>>>> Fixes: commit 3a2d7fa8a3d5 ("mm: disable interrupts while initializing
>>>> deferred pages")
>>>>
>>>> Signed-off-by: Shile Zhang <shile.zhang@linux.alibaba.com>
>>>> ---
>>>>    include/linux/memory_hotplug.h | 16 ++++++++++++++--
>>>>    1 file changed, 14 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
>>>> index ba0dca6aac6e..be69a6dc4fee 100644
>>>> --- a/include/linux/memory_hotplug.h
>>>> +++ b/include/linux/memory_hotplug.h
>>>> @@ -6,6 +6,8 @@
>>>>    #include <linux/spinlock.h>
>>>>    #include <linux/notifier.h>
>>>>    #include <linux/bug.h>
>>>> +#include <linux/sched.h>
>>>> +#include <linux/smp.h>
>>>>      struct page;
>>>>    struct zone;
>>>> @@ -282,12 +284,22 @@ static inline bool movable_node_is_enabled(void)
>>>>    static inline
>>>>    void pgdat_resize_lock(struct pglist_data *pgdat, unsigned long *flags)
>>>>    {
>>>> -    spin_lock_irqsave(&pgdat->node_size_lock, *flags);
>>>> +    /*
>>>> +     * Disable local interrupts on boot CPU will stop the tick_sched
>>>> +     * timer, which will block jiffies(wall clock) update.
>>>> +     */
>>>> +    if (current->cpu != get_boot_cpu_id())
>>>> +        spin_lock_irqsave(&pgdat->node_size_lock, *flags);
>>>> +    else
>>>> +        spin_lock(&pgdat->node_size_lock);
>>>>    }
>>>>    static inline
>>>>    void pgdat_resize_unlock(struct pglist_data *pgdat, unsigned long *flags)
>>>>    {
>>>> -    spin_unlock_irqrestore(&pgdat->node_size_lock, *flags);
>>>> +    if (current->cpu != get_boot_cpu_id())
>>>> +        spin_unlock_irqrestore(&pgdat->node_size_lock, *flags);
>>>> +    else
>>>> +        spin_unlock(&pgdat->node_size_lock);
>>>>    }
>>>>    static inline
>>>>    void pgdat_resize_init(struct pglist_data *pgdat)
>>> 1)Linux kernel is *preemptible*. Kernel with CONFIG_PREEMPT_RT option even may preempt
>>> *kernel* code in the middle of function. When you are executing a code containing
>>> pgdat_resize_lock() and pgdat_resize_unlock(), the process may migrate to another cpu
>>> between them.
>>>
>>> bool cpu               another cpu
>>> ----------------------------------
>>> pgdat_resize_lock()
>>>     spin_lock()
>>>     --> migrate to another cpu
>>>                         pgdat_resize_unlock()
>>>                         spin_unlock_irqrestore(<uninitialized flags>)
>>>
>>> (Yes, in case of CONFIG_PREEMPT_RT, process is preemptible even after spin_lock() call).
>>>
>>> This looks like a bad helpers, and we should not introduce such the design.
>> Hi Kirill,
>>
>> Thanks for your comments!
>> Sorry for I'm not very clear about this lock/unlock, but I encountered this issue
>> with "CONFIG_PREEMPT is not set".
> The thing is we simply shouldn't introduce such the primitives since the thread
> may migrate to another cpu, while you own the lock. This looks like a buggy design.
>
>>> 2)I think there is no the problem this patch solves. Do we really this statistics?
>>> Can't we simple remove print message from deferred_init_memmap() and solve this?
>> Sorry for I've not put this issue very clearly. It's *not* just one statistics log
>> with wrong time calculate, but the wall clock is stuck.
>> So the 'systemd-analyze' command also give a wrong time as I mentioned in the cover
>> letter. I don't think is OK just remove the log, it cannot solve the wall clock latency.
> Have you tried temporary enabling interrupts in the middle of cycle after a huge enough
> memory block is initialized? Something like:
>
> deferred_init_memmap()
> {
> 	while (spfn < epfn) {
> 		nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
> 		local_irq_enable();
> 		local_irq_disable();
> 	}
> }

Yes, I'd tried this for issue confirm, before I sent this patch. Likes I 
mentioned the debug log in cover letter, I also add mdelay between 
local_irq_enable/disable, this system jiffies is stuck without update.
So I think there must be problem to use spin_lock_irqsave in early boot 
path on boot CPU.
> Or, maybe, enable/disable interrupts somewhere inside deferred_init_maxorder().
>
>>> Also, you may try to check that sched_clock() gives better results with interrupts
>>> disabled (on x86 it uses rdtsc, when it's possible. But it also may fallback to
>>> jiffies-based clock in some hardware cases, and they also won't go with interrupts
>>> disabled).