Re: [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@xxxxxxxxxxxxxxxxx>
---
  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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