[PATCH 2/2] arm64: validate the delta of cycle_now and cycle_last

Ding Tianhong dingtianhong at huawei.com
Tue Oct 27 18:33:39 PDT 2015 

On 2015/10/27 22:03, Mark Rutland wrote:
> On Tue, Oct 27, 2015 at 09:21:13PM +0800, Yang Yingliang wrote:
>> 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?
> 
Hi Mark:

we have check this with the chip developer, it is a hardware design problem, because there is
too much cores in the chip, and we could not sure all the chip from several company could totally
fix this problem, the x86 still has this problem, so I think this patch is need for
arm64 to compatible the different chip.

Ding

>> 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?
> 
> How long does it take to trigger the issue?
> 
>> 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?
> 
>> 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.
> 
>> 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
> 
>>
>> Signed-off-by: Yang Yingliang <yangyingliang at huawei.com>
>> Cc: Thomas Gleixner <tglx at linutronix.de>
>> ---
>>  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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>>
> 
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