Re: [KVM] e923b0537d: kernel-selftests.kvm.rseq_test.fail

[Gavin Shan]

Hi Sean,

On 8/16/22 3:02 PM, Gavin Shan wrote:
> On 8/16/22 7:42 AM, Sean Christopherson wrote:
> > On Mon, Aug 15, 2022, kernel test robot wrote:
> > > commit: e923b0537d28e15c9d31ce8b38f810b325816903 ("KVM: selftests: Fix target thread to be migrated in rseq_test")
> > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> >
> > ...
> >
> > > # selftests: kvm: rseq_test
> > > # ==== Test Assertion Failure ====
> > > #   rseq_test.c:278: i > (NR_TASK_MIGRATIONS / 2)
> > > #   pid=49599 tid=49599 errno=4 - Interrupted system call
> > > #      1    0x000000000040265d: main at rseq_test.c:278
> > > #      2    0x00007fe44eed07fc: ?? ??:0
> > > #      3    0x00000000004026d9: _start at ??:?
> > > #   Only performed 23174 KVM_RUNs, task stalled too much?
> > > #
> > > not ok 56 selftests: kvm: rseq_test # exit=254
> >
> > ...
> >
> > > # Automatically generated file; DO NOT EDIT.
> > > # Linux/x86_64 5.19.0-rc6 Kernel Configuration
> > > #
> > > CONFIG_CC_VERSION_TEXT="gcc-11 (Debian 11.3.0-3) 11.3.0"
> > > CONFIG_CC_IS_GCC=y
> > > CONFIG_GCC_VERSION=110300
> > > CONFIG_CLANG_VERSION=0
> > > CONFIG_AS_IS_GNU=y
> > > CONFIG_AS_VERSION=23800
> > > CONFIG_LD_IS_BFD=y
> > > CONFIG_LD_VERSION=23800
> > > CONFIG_LLD_VERSION=0
> >
> > Assuming 23800 == 2.38, this is a known issue.
> >
> > https://lore.kernel.org/all/20220810104114.6838-1-gshan@xxxxxxxxxx
>
> It's probably different story this time. The assert is triggered because
> of the following instructions. I would guess the reason is vcpu thread
> has been running on CPU where we has high CPU load. In this case, the
> vcpu thread can't be run in time. More specific, the vcpu thread can't
> be run in the 1 - 10us time window, which is specified by the migration
> worker (thread).
>
>      TEST_ASSERT(i > (NR_TASK_MIGRATIONS / 2),
>                  "Only performed %d KVM_RUNs, task stalled too much?\n", i);
>
> I think we need to improve the handshake mechanism between the vcpu thread
> and migration worker. In current implementation, the handshake is done through
> the atomic counter. The mechanism is simple enough, but vcpu thread can miss
> the aforementioned time window. Another issue is the test case much more time
> than expected to finish.
>
> Sean, if you think it's reasonable, I can figure out something to do:
>
> - Reuse the atomic counter for a full synchronization between these two
>    threads. Something like below:
>
>    #define RSEQ_TEST_STATE_RUN_VCPU       0     // vcpu_run()
>    #define RSEQ_TEST_STATE_MIGRATE        1     // sched_setaffinity()
>    #define RSEQ_TEST_STATE_CHECK          2     // Check rseq.cpu_id and get_cpu()
>
>    The atomic counter is reset to RSEQ_TEST_STATE_RUN_VCPU after RSEQ_TEST_STATE_RUN_VCPU
>
> - Reduce NR_TASK_MIGRATIONS from 100000 to num_of_online_cpus(). With this,
>    less time is needed to finish the test case.

I'm able to recreate the issue on my local arm64 system.

- From the source code, the iteration count is changed from 100000 to 1000
- Only CPU#0 and CPU#1 are exposed in calc_min_max_cpu, meaning other CPUs
  are cleared from @possible_mask
- Run some CPU bound task on CPU#0 and CPU#1
  # while true; do taskset -c 0 ./a; done
  # while true; do taskset -c 1 ./a; done
- Run 'rseq_test' and hit the issue
  # ./rseq_test
  calc_min_max_cpu: nproc=224
  calc_min_max_cpu: min_cpu=0, max_cpu=1
  main: Required tests: 1000   Succeeds: 1
  ==== Test Assertion Failure ====
    rseq_test.c:280: i > (NR_TASK_MIGRATIONS / 2)
    pid=9624 tid=9624 errno=4 - Interrupted system call
       1	0x0000000000401cf3: main at rseq_test.c:280
       2	0x0000ffffbc64679b: ?? ??:0
       3	0x0000ffffbc64686b: ?? ??:0
       4	0x0000000000401def: _start at ??:?
    Only performed 1 KVM_RUNs, task stalled too much?

Thanks,
Gavin