Re: [PATCH] tests/functional: fix race in virtio balloon test

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Mar 04, 2025 at 06:33:40PM +0000, Daniel P. Berrangé wrote:
> There are two race conditions in the recently added virtio balloon
> test
> 
>  * The /dev/vda device node is not ready
>  * The virtio-balloon driver has not issued the first stats refresh
> 
> To fix the former, monitor dmesg for a line about 'vda'.
> 
> To fix the latter, retry the stats query until seeing fresh data.
> 
> Adding 'quiet' to the kernel command line reduces serial output
> which otherwise slows boot, making it less likely to hit the former
> race too.
> 
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

ok

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  tests/functional/test_virtio_balloon.py | 24 +++++++++++++++++++-----
>  1 file changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/tests/functional/test_virtio_balloon.py b/tests/functional/test_virtio_balloon.py
> index 67b48e1b4e..308d197eb3 100755
> --- a/tests/functional/test_virtio_balloon.py
> +++ b/tests/functional/test_virtio_balloon.py
> @@ -32,7 +32,7 @@ class VirtioBalloonx86(QemuSystemTest):
>          'e3c1b309d9203604922d6e255c2c5d098a309c2d46215d8fc026954f3c5c27a0')
>  
>      DEFAULT_KERNEL_PARAMS = ('root=/dev/vda1 console=ttyS0 net.ifnames=0 '
> -                             'rd.rescue')
> +                             'rd.rescue quiet')
>  
>      def wait_for_console_pattern(self, success_message, vm=None):
>          wait_for_console_pattern(
> @@ -47,6 +47,9 @@ def mount_root(self):
>          prompt = '# '
>          self.wait_for_console_pattern(prompt)
>  
> +        # Synchronize on virtio-block driver creating the root device
> +        exec_command_and_wait_for_pattern(self, "while ! (dmesg -c | grep vda:) ; do sleep 1 ; done", "vda1")
> +
>          exec_command_and_wait_for_pattern(self, 'mount /dev/vda1 /sysroot',
>                                            prompt)
>          exec_command_and_wait_for_pattern(self, 'chroot /sysroot',
> @@ -65,10 +68,21 @@ def assert_initial_stats(self):
>              assert val == UNSET_STATS_VALUE
>  
>      def assert_running_stats(self, then):
> -        ret = self.vm.qmp('qom-get',
> -                          {'path': '/machine/peripheral/balloon',
> -                           'property': 'guest-stats'})['return']
> -        when = ret.get('last-update')
> +        # We told the QEMU to refresh stats every 100ms, but
> +        # there can be a delay between virtio-ballon driver
> +        # being modprobed and seeing the first stats refresh
> +        # Retry a few times for robustness under heavy load
> +        retries = 10
> +        when = 0
> +        while when == 0 and retries:
> +            ret = self.vm.qmp('qom-get',
> +                              {'path': '/machine/peripheral/balloon',
> +                               'property': 'guest-stats'})['return']
> +            when = ret.get('last-update')
> +            if when == 0:
> +                retries = retries - 1
> +                time.sleep(0.5)
> +
>          now = time.time()
>  
>          assert when > then and when < now
> -- 
> 2.48.1