linux-kernel - Re: [PATCH v3] KVM: X86: Fix softlockup when get the current kvmclock

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Date:   Thu, 9 Nov 2017 13:47:13 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v3] KVM: X86: Fix softlockup when get the current kvmclock

2017-11-09 12:39 GMT+08:00 Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>:
> On Wed, Nov 08, 2017 at 06:52:12PM -0800, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@...mail.com>
>>
>>  watchdog: BUG: soft lockup - CPU#6 stuck for 22s! [qemu-system-x86:10185]
>>  CPU: 6 PID: 10185 Comm: qemu-system-x86 Tainted: G           OE   4.14.0-rc4+ #4
>>  RIP: 0010:kvm_get_time_scale+0x4e/0xa0 [kvm]
>>  Call Trace:
>>   ? get_kvmclock_ns+0xa3/0x140 [kvm]
>>   get_time_ref_counter+0x5a/0x80 [kvm]
>>   kvm_hv_process_stimers+0x120/0x5f0 [kvm]
>>   ? kvm_hv_process_stimers+0x120/0x5f0 [kvm]
>>   ? preempt_schedule+0x27/0x30
>>   ? ___preempt_schedule+0x16/0x18
>>   kvm_arch_vcpu_ioctl_run+0x4b4/0x1690 [kvm]
>>   ? kvm_arch_vcpu_load+0x47/0x230 [kvm]
>>   kvm_vcpu_ioctl+0x33a/0x620 [kvm]
>>   ? kvm_vcpu_ioctl+0x33a/0x620 [kvm]
>>   ? kvm_vm_ioctl_check_extension_generic+0x3b/0x40 [kvm]
>>   ? kvm_dev_ioctl+0x279/0x6c0 [kvm]
>>   do_vfs_ioctl+0xa1/0x5d0
>>   ? __fget+0x73/0xa0
>>   SyS_ioctl+0x79/0x90
>>   entry_SYSCALL_64_fastpath+0x1e/0xa9
>>
>> This can be reproduced when running kvm-unit-tests/hyperv_stimer.flat and
>> cpu-hotplug stress simultaneously. __this_cpu_read(cpu_tsc_khz) returns 0
>> (set in kvmclock_cpu_down_prep()) when the pCPU is unhotplug which results
>> in kvm_get_time_scale() gets into an infinite loop.
>>
>> This patch fixes it by treating the unhotplug pCPU as not using master clock.
>>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>> ---
>>  arch/x86/kvm/x86.c | 11 +++++++----
>>  1 file changed, 7 insertions(+), 4 deletions(-)
>
> Could you include the v1, v2, and v3 changes? I kind got lost
> why the earlier versions were incorrect.
>
> Usually one does it by saying:
> v1: New version
> v2: Changed xyz , etc..

Yeah, I forgot to add this:

v2 -> v3:
 * treat the unhotplug pCPU as not using master clock
v1 -> v2:
 * avoid infinite loop in kvm_get_time_scale()

Regards,
Wanpeng Li

>
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 03869eb..d61dcce3 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1795,10 +1795,13 @@ u64 get_kvmclock_ns(struct kvm *kvm)
>>       /* both __this_cpu_read() and rdtsc() should be on the same cpu */
>>       get_cpu();
>>
>> -     kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
>> -                        &hv_clock.tsc_shift,
>> -                        &hv_clock.tsc_to_system_mul);
>> -     ret = __pvclock_read_cycles(&hv_clock, rdtsc());
>> +     if (__this_cpu_read(cpu_tsc_khz)) {
>> +             kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
>> +                                &hv_clock.tsc_shift,
>> +                                &hv_clock.tsc_to_system_mul);
>> +             ret = __pvclock_read_cycles(&hv_clock, rdtsc());
>> +     } else
>> +             ret = ktime_get_boot_ns() + ka->kvmclock_offset;
>>
>>       put_cpu();
>>
>> --
>> 2.7.4
>>