linux-kernel - Re: [PATCH v3] KVM: selftests: Fix target thread to be migrated in rseq

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Date:   Tue, 19 Jul 2022 12:13:32 +1000
From:   Gavin Shan <gshan@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, oliver.upton@...ux.dev,
        shuah@...nel.org, maz@...nel.org, pbonzini@...hat.com,
        shan.gavin@...il.com
Subject: Re: [PATCH v3] KVM: selftests: Fix target thread to be migrated in
 rseq_test

Hi Sean,

On 7/19/22 9:46 AM, Sean Christopherson wrote:
> On Tue, Jul 19, 2022, Gavin Shan wrote:
>> ---
>> v3: Improved changelog (Oliver Upon)
> 
> Sorry I didn't catch v3, I saw that you waited but just didn't get to this earlier :-/
> 

Not a problem at all :)

>> ---
>>   tools/testing/selftests/kvm/rseq_test.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/testing/selftests/kvm/rseq_test.c b/tools/testing/selftests/kvm/rseq_test.c
>> index 4158da0da2bb..c83ac7b467f8 100644
>> --- a/tools/testing/selftests/kvm/rseq_test.c
>> +++ b/tools/testing/selftests/kvm/rseq_test.c
>> @@ -38,6 +38,7 @@ static __thread volatile struct rseq __rseq = {
>>    */
>>   #define NR_TASK_MIGRATIONS 100000
>>   
>> +static pid_t rseq_tid;
>>   static pthread_t migration_thread;
>>   static cpu_set_t possible_mask;
>>   static int min_cpu, max_cpu;
>> @@ -106,7 +107,8 @@ static void *migration_worker(void *ign)
> 
> Pass the target TID to the worker, then there's no need to use a global and no
> chance of consuming rseq_tid "uninitialized".  The casting to convert gettid() to
> a "void *" is annoying, but not the end of the world.
> 

I was thinking of the scheme, but passing the address of a local variable
for the thread ID. Your suggestion also makes sense to me.

>>   		 * stable, i.e. while changing affinity is in-progress.
>>   		 */
>>   		smp_wmb();
>> -		r = sched_setaffinity(0, sizeof(allowed_mask), &allowed_mask);
>> +		r = sched_setaffinity(rseq_tid, sizeof(allowed_mask),
>> +				      &allowed_mask);
> 
> Eh, let this poke out, don't think it's worth wrapping here.
> 

Ok, I was trying to follow rule of 80-characters per line, even it's
not strictly needed nowadays. It's also fine not to follow :)

I just picked your code and posted v4:

https://lore.kernel.org/kvmarm/20220719020830.3479482-1-gshan@redhat.com/T/#u

Thanks,
Gavin

> E.g.
> 
> ---
>   tools/testing/selftests/kvm/rseq_test.c | 8 +++++---
>   1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/rseq_test.c b/tools/testing/selftests/kvm/rseq_test.c
> index aba7be178dab..a54d4d05a058 100644
> --- a/tools/testing/selftests/kvm/rseq_test.c
> +++ b/tools/testing/selftests/kvm/rseq_test.c
> @@ -80,8 +80,9 @@ static int next_cpu(int cpu)
>   	return cpu;
>   }
> 
> -static void *migration_worker(void *ign)
> +static void *migration_worker(void *__rseq_tid)
>   {
> +	pid_t rseq_tid = (pid_t)(unsigned long)__rseq_tid;
>   	cpu_set_t allowed_mask;
>   	int r, i, cpu;
> 
> @@ -104,7 +105,7 @@ static void *migration_worker(void *ign)
>   		 * stable, i.e. while changing affinity is in-progress.
>   		 */
>   		smp_wmb();
> -		r = sched_setaffinity(0, sizeof(allowed_mask), &allowed_mask);
> +		r = sched_setaffinity(rseq_tid, sizeof(allowed_mask), &allowed_mask);
>   		TEST_ASSERT(!r, "sched_setaffinity failed, errno = %d (%s)",
>   			    errno, strerror(errno));
>   		smp_wmb();
> @@ -227,7 +228,8 @@ int main(int argc, char *argv[])
>   	vm = vm_create_with_one_vcpu(&vcpu, guest_code);
>   	ucall_init(vm, NULL);
> 
> -	pthread_create(&migration_thread, NULL, migration_worker, 0);
> +	pthread_create(&migration_thread, NULL, migration_worker,
> +		       (void *)(unsigned long)gettid());
> 
>   	for (i = 0; !done; i++) {
>   		vcpu_run(vcpu);
> 
> base-commit: ad6cb756bb497997032df2bda7cbdff076e4a66a
> --
>