On 2015/10/27 22:03, Mark Rutland wrote:

On Tue, Oct 27, 2015 at 09:21:13PM +0800, Yang Yingliang wrote:

quoted

In multi-core system, if the clock is not sync perfectly, it
will make cycle_last that recorded by CPU-A is a little more
than cycle_now that read by CPU-B.

If that is happening, that sounds like a hardware and/or firmware bug.

The ARM ARM states the following (where a CPU is a device):

	The system counter must provide a uniform view of system time. More
	precisely, it must be impossible for the following sequence of events
	to show system time going backwards:

	1. Device A reads the time from the system counter.

	2. Device A communicates with another agent in the system, Device B.

	3. After recognizing the communication from Device A, Device B reads
	   the time from the system counter.

Per [1] it seems like the TSC is not architected to provide this guarantee for
x86.

Are you certain that the CPUs have clocks which are not in sync?

quoted

With the negative result,
hrtimer_update_base() return a huge and wrong time. It leads
to the cpu can not finish the while loop in hrtimer_interrupt()
until the real nowtime which is returned from ktime_get() catch
up with the wrong time on clock monotonic base.

I was able to reproudce the problem with calling clock_settime
and clock_adjtime repeatedly on each cpu. The params of the calls
is random.

Could you share your reproducer?

https://github.com/kernelslacker/trinity

I use this testsuite and I make it call clock_settime and
clock_adjtime alternately with random params on each core.

How long does it take to trigger the issue?

It's not certain. It would take half an hour or several hours or more 
longer.

quoted

Here is the calltrace:

Jan 01 00:02:29 Linux kernel: INFO: rcu_sched detected stalls on CPUs/tasks:
Jan 01 00:02:29 Linux kernel:         0: (2 GPs behind) idle=913/1/0 softirq=59289/59291 fqs=488
Jan 01 00:02:29 Linux kernel:         (detected by 20, t=5252 jiffies, g=35769, c=35768, q=567)
Jan 01 00:02:29 Linux kernel: Task dump for CPU 0:
Jan 01 00:02:29 Linux kernel: swapper/0       R  running task        0   0      0 0x00000002
Jan 01 00:02:29 Linux kernel: Call trace:
Jan 01 00:02:29 Linux kernel: [<ffffffc000086c5c>] __switch_to+0x74/0x8c
Jan 01 00:02:29 Linux kernel: rcu_sched kthread starved for 4764 jiffies!
Jan 01 00:03:32 Linux kernel: NMI watchdog: BUG: soft lockup - CPU#0 stuck for 23s! [swapper/0:0]
Jan 01 00:03:32 Linux kernel: CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.1.6+ #184
Jan 01 00:03:32 Linux kernel: task: ffffffc00091cdf0 ti: ffffffc000910000 task.ti: ffffffc000910000
Jan 01 00:03:32 Linux kernel: PC is at arch_cpu_idle+0x10/0x18
Jan 01 00:03:32 Linux kernel: LR is at arch_cpu_idle+0xc/0x18
Jan 01 00:03:32 Linux kernel: pc : [<ffffffc00008686c>] lr : [<ffffffc000086868>] pstate: 60000145
Jan 01 00:03:32 Linux kernel: sp : ffffffc000913f20
Jan 01 00:03:32 Linux kernel: x29: ffffffc000913f20 x28: 000000003f4bbab0
Jan 01 00:03:32 Linux kernel: x27: ffffffc00091669c x26: ffffffc00096e000
Jan 01 00:03:32 Linux kernel: x25: ffffffc000804000 x24: ffffffc000913f30
Jan 01 00:03:32 Linux kernel: x23: 0000000000000001 x22: ffffffc0006817f8
Jan 01 00:03:32 Linux kernel: x21: ffffffc0008fdb00 x20: ffffffc000916618
Jan 01 00:03:32 Linux kernel: x19: ffffffc000910000 x18: 00000000ffffffff
Jan 01 00:03:32 Linux kernel: x17: 0000007f9d6f682c x16: ffffffc0001e19d0
Jan 01 00:03:32 Linux kernel: x15: 0000000000000061 x14: 0000000000000072
Jan 01 00:03:32 Linux kernel: x13: 0000000000000067 x12: ffffffc000682528
Jan 01 00:03:32 Linux kernel: x11: 0000000000000005 x10: 00000001000faf9a
Jan 01 00:03:32 Linux kernel: x9 : ffffffc000913e60 x8 : ffffffc00091d350
Jan 01 00:03:32 Linux kernel: x7 : 0000000000000000 x6 : 002b24c4f00026aa
Jan 01 00:03:32 Linux kernel: x5 : 0000001ffd5c6000 x4 : ffffffc000913ea0
Jan 01 00:03:32 Linux kernel: x3 : ffffffdffdec3b44 x2 : ffffffdffdec3b44
Jan 01 00:03:32 Linux kernel: x1 : 0000000000000000 x0 : 0000000000000000

In this case was CNTVOFF uniform on all CPUs?

Was the kernel booted at EL2 or EL1N? Was it booted under a hypervisor?

At EL1N without a hypervisor.

quoted

CPU-A updates the cycle_last in do_settimeofday64() under lock and CPU-B
reads the current cycles which is slightly behind CPU-A to substract the
cycle_last after unlock, then we get a negative result, after masking it
comes to a extremely huge value and lead to "hang" in hrtimer_interrupt().

It's possible that we have missing ISBs or DSBs somewhere, and we're
encountering a re-ordering that we did not expect.

It would be very helpful to be able to analyse with your reproducer. I
have a kernel test in a local branch which I've never managed to trigger
a failure with otehr than on systems with a horrifically skewed CNTVOFF.

quoted

And multi-core system on X86 had already met such problem and Thomas introduce
a fix which is commit 47001d603375 ("x86: tsc prevent time going backwards").
And then Thomas moved the fix code into the core code file of time in
commit 09ec54429c6d ("clocksource: Move cycle_last validation to core code").
Now the validation can be enabled by config CLOCKSOURCE_VALIDATE_LAST_CYCLE.
I think we can fix the problem on arm64 by selecting the config. This is no
side effect for systems with counters running properly.

As above, per [1], I'm not sure that the same rationale applies, and
it's somewhat worrying to mask the issue in this manner.

Thanks,
Mark.

[1] https://lkml.org/lkml/2007/8/23/96

quoted

Signed-off-by: Yang Yingliang <redacted>
Cc: Thomas Gleixner <redacted>
---
  arch/arm64/Kconfig | 1 +
  1 file changed, 1 insertion(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 07d1811..6a53926 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -30,6 +30,7 @@ config ARM64
  	select GENERIC_ALLOCATOR
  	select GENERIC_CLOCKEVENTS
  	select GENERIC_CLOCKEVENTS_BROADCAST
+	select CLOCKSOURCE_VALIDATE_LAST_CYCLE
  	select GENERIC_CPU_AUTOPROBE
  	select GENERIC_EARLY_IOREMAP
  	select GENERIC_IDLE_POLL_SETUP
--

2.5.0



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