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>
---
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).
