Re: [PATCH 1/2] sched/core: Enable full cpumask to clear user cpumask in sched_setaffinity()

[Waiman Long <llong@redhat.com>]

On 10/20/25 4:13 PM, David Hildenbrand wrote:
> On 23.09.25 19:54, Waiman Long wrote:
>> Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
>> cpumask"), user provided CPU affinity via sched_setaffinity(2) is
>> perserved even if the task is being moved to a different cpuset.
>> However, that affinity is also being inherited by any subsequently
>> created child processes which may not want or be aware of that affinity.
>
> So I assume setting the affinity to the full bitmap would then allow 
> any child to essentially reset to the default, correct?
Yes, that is the point.
>
>>
>> One way to solve this problem is to provide a way to back off from
>> that user provided CPU affinity.  This patch implements such a scheme
>> by using a full cpumask (a cpumask with all bits set) to signal the
>> clearing of the user cpumask to follow the default as allowed by
>> the current cpuset.  In fact, with a full cpumask in user_cpus_ptr,
>> the task behavior should be the same as with a NULL user_cpus_ptr.
>> This patch just formalizes it without causing any incompatibility and
>> discard an otherwise useless cpumask.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   kernel/sched/syscalls.c | 20 ++++++++++++++------
>>   1 file changed, 14 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
>> index 77ae87f36e84..d68c7a4ee525 100644
>> --- a/kernel/sched/syscalls.c
>> +++ b/kernel/sched/syscalls.c
>> @@ -1229,14 +1229,22 @@ long sched_setaffinity(pid_t pid, const 
>> struct cpumask *in_mask)
>>           return retval;
>>         /*
>> -     * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
>> -     * alloc_user_cpus_ptr() returns NULL.
>> +     * If a full cpumask is passed in, clear user_cpus_ptr and reset 
>> the
>> +     * current cpu affinity to the default for the current cpuset.
>>        */
>> -    user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
>> -    if (user_mask) {
>> -        cpumask_copy(user_mask, in_mask);
>> +    if (cpumask_full(in_mask)) {
>> +        user_mask = NULL;
>>       } else {
>> -        return -ENOMEM;
>> +        /*
>> +         * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
>> +         * alloc_user_cpus_ptr() returns NULL.
>> +         */
>> +        user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
>> +        if (user_mask) {
>> +            cpumask_copy(user_mask, in_mask);
>> +        } else {
>> +            return -ENOMEM;
>> +        }
>>       }
>>         ac = (struct affinity_context){
>
> Not an expert on this code.
>
> I'm only wondering if there is somehow, some way we could be breaking 
> user space by doing that.
>
I don't think so. Setting user_cpus_ptr to a full cpumask will make the 
task strictly follow the cpumask restriction imposed by the current 
cpuset as if user_cpus_ptr isn't set.

Cheers,
Longman