Messages in this thread | | | Subject | Re: [PATCH RESEND] mm: fix tick_sched timer blocked by pgdat_resize_lock | From | Shile Zhang <> | 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).
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