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SubjectRe: [PATCH RESEND] mm: fix tick_sched timer blocked by pgdat_resize_lock
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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".
> 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.
> 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).
>
> Kirill

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