Re: [PATCH RESEND] mm: fix tick_sched timer blocked by pgdat_resize_lock

[Shile Zhang]

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@xxxxxxxxxxxxxxxxx>
> > > > ---
> > > >  Â include/linux/memory_hotplug.h | 16 ++++++++++++++--
> > > >  Â 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 @@
> > > >  Â #include <linux/spinlock.h>
> > > >  Â #include <linux/notifier.h>
> > > >  Â #include <linux/bug.h>
> > > > +#include <linux/sched.h>
> > > > +#include <linux/smp.h>
> > > >  Â Â struct page;
> > > >  Â struct zone;
> > > > @@ -282,12 +284,22 @@ static inline bool movable_node_is_enabled(void)
> > > >  Â static inline
> > > >  Â void pgdat_resize_lock(struct pglist_data *pgdat, unsigned long *flags)
> > > >  Â {
> > > > -ÂÂÂ spin_lock_irqsave(&pgdat->node_size_lock, *flags);
> > > > +ÂÂÂ /*
> > > > +ÂÂÂÂ * Disable local interrupts on boot CPU will stop the tick_sched
> > > > +ÂÂÂÂ * timer, which will block jiffies(wall clock) update.
> > > > +ÂÂÂÂ */
> > > > +ÂÂÂ if (current->cpu != get_boot_cpu_id())
> > > > +ÂÂÂÂÂÂÂ spin_lock_irqsave(&pgdat->node_size_lock, *flags);
> > > > +ÂÂÂ else
> > > > +ÂÂÂÂÂÂÂ spin_lock(&pgdat->node_size_lock);
> > > >  Â }
> > > >  Â static inline
> > > >  Â void pgdat_resize_unlock(struct pglist_data *pgdat, unsigned long *flags)
> > > >  Â {
> > > > -ÂÂÂ spin_unlock_irqrestore(&pgdat->node_size_lock, *flags);
> > > > +ÂÂÂ if (current->cpu != get_boot_cpu_id())
> > > > +ÂÂÂÂÂÂÂ spin_unlock_irqrestore(&pgdat->node_size_lock, *flags);
> > > > +ÂÂÂ else
> > > > +ÂÂÂÂÂÂÂ spin_unlock(&pgdat->node_size_lock);
> > > >  Â }
> > > >  Â static inline
> > > >  Â 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 cpuÂÂÂÂÂÂÂÂÂÂÂÂÂÂ another cpu
> > > ----------------------------------
> > > pgdat_resize_lock()
> > >  ÂÂ spin_lock()
> > >  ÂÂ --> migrate to another cpu
> > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ pgdat_resize_unlock()
> > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 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).