Messages in this thread |  | | Subject | Re: [PATCH RESEND] mm: fix tick_sched timer blocked by pgdat_resize_lock | From | Shile Zhang <> | Date | Tue, 4 Feb 2020 15:24:38 +0800 |
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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@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. > 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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