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Date: Tue, 4 Feb 2020 15:24:38 +0800
From: Shile Zhang <shile.zhang@...ux.alibaba.com>
To: Kirill Tkhai <ktkhai@...tuozzo.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Pavel Tatashin <pasha.tatashin@...een.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] mm: fix tick_sched timer blocked by
pgdat_resize_lock
Hi, Andrew,
Sorry for ping, could you please also help to check this issue?
Any comments from you is welcome!
Thanks!
On 2020/1/15 17:45, Kirill Tkhai wrote:
> On 14.01.2020 11:54, Shile Zhang wrote:
>>
>> 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@...ux.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.
> This time SMP is enabled. You have many threads are running. Interrupts are enabled
> and they occur. So, it's OK to disable interrupts for some time.
>
> My opinion is we should try to enable interrupts in the cycle after some fixed
> amount of memory is initialized. Say, every 1GB. This should resolve two problems:
> handling timer interrupt with update jiffies at time, and keeping the fix for the issue,
> that Pavel fixed in 3a2d7fa8a3d5.
>
>>> 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).
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