Re: [PATCH 2/2] selftests: kvm: Reduce verbosity of "Random seed" messages

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 6/21/2024 2:13 AM, Sean Christopherson wrote:
> On Thu, Jun 20, 2024, Dapeng Mi wrote:
>> Huge number of "Random seed:" messages are printed when running
>> pmu_counters_test. It leads to the regular test output is totally
>> flooded by these over-verbose messages.
>>
>> Downgrade "Random seed" message printing level to debug and prevent it
>> to be printed in normal case.
> I completely agree this is annoying, but the whole point of printing the seed is
> so that the seed is automatically captured if a test fails, e.g. so that the
> failure can be reproduced if it is dependent on some random decision.
>
> Rather than simply hiding the message, what if print the seed if and only if it
> changes?

Yeah, it's indeed better.

>
> --
> From: Sean Christopherson <seanjc@google.com>
> Date: Thu, 20 Jun 2024 10:29:53 -0700
> Subject: [PATCH] KVM: selftests: Print the seed for the guest pRNG iff it has
>  changed

s/iff/if/


>
> Print the guest's random seed during VM creation if and only if the seed
> has changed since the seed was last printed.  The vast majority of tests,
> if not all tests at this point, set the seed during test initialization
> and never change the seed, i.e. printing it every time a VM is created is
> useless noise.
>
> Snapshot and print the seed during early selftest init to play nice with
> tests that use the kselftests harness, at the cost of printing an unused
> seed for tests that change the seed during test-specific initialization,
> e.g. dirty_log_perf_test.  The kselftests harness runs each testcase in a
> separate process that is forked from the original process before creating
> each testcase's VM, i.e. waiting until first VM creation will result in
> the seed being printed by each testcase despite it never changing.  And
> long term, the hope/goal is that setting the seed will be handled by the
> core framework, i.e. that the dirty_log_perf_test wart will naturally go
> away.
>
> Reported-by: Yi Lai <yi1.lai@intel.com>
> Reported-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  tools/testing/selftests/kvm/lib/kvm_util.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index ad00e4761886..56b170b725b3 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> @@ -21,6 +21,7 @@
>  
>  uint32_t guest_random_seed;
>  struct guest_random_state guest_rng;
> +static uint32_t last_guest_seed;
>  
>  static int vcpu_mmap_sz(void);
>  
> @@ -434,7 +435,10 @@ struct kvm_vm *__vm_create(struct vm_shape shape, uint32_t nr_runnable_vcpus,
>  	slot0 = memslot2region(vm, 0);
>  	ucall_init(vm, slot0->region.guest_phys_addr + slot0->region.memory_size);
>  
> -	pr_info("Random seed: 0x%x\n", guest_random_seed);
> +	if (guest_random_seed != last_guest_seed) {
> +		pr_info("Random seed: 0x%x\n", guest_random_seed);
> +		last_guest_seed = guest_random_seed;
> +	}
>  	guest_rng = new_guest_random_state(guest_random_seed);
>  	sync_global_to_guest(vm, guest_rng);
>  
> @@ -2319,7 +2323,8 @@ void __attribute((constructor)) kvm_selftest_init(void)
>  	/* Tell stdout not to buffer its content. */
>  	setbuf(stdout, NULL);
>  
> -	guest_random_seed = random();
> +	guest_random_seed = last_guest_seed = random();
> +	pr_info("Random seed: 0x%x\n", guest_random_seed);
>  
>  	kvm_selftest_arch_init();
>  }
>
> base-commit: c81b138d5075c6f5ba3419ac1d2a2e7047719c14