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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Nov 19, 2020 at 09:47:44AM +0000, Quentin Perret wrote: > On Friday 13 Nov 2020 at 09:37:16 (+0000), Will Deacon wrote: > > Reject explicit requests to change the affinity mask of a task via > > set_cpus_allowed_ptr() if the requested mask is not a subset of the > > mask returned by arch_cpu_allowed_mask(). This ensures that the > > 'cpus_mask' for a given task cannot contain CPUs which are incapable of > > executing it, except in cases where the affinity is forced. > > > > Signed-off-by: Will Deacon > > --- > > kernel/sched/core.c | 4 ++++ > > 1 file changed, 4 insertions(+) > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index 8df38ebfe769..13bdb2ae4d3f 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -1877,6 +1877,7 @@ static int __set_cpus_allowed_ptr_locked(struct task_struct *p, > > struct rq_flags *rf) > > { > > const struct cpumask *cpu_valid_mask = cpu_active_mask; > > + const struct cpumask *cpu_allowed_mask = arch_cpu_allowed_mask(p); > > unsigned int dest_cpu; > > int ret = 0; > > > > @@ -1887,6 +1888,9 @@ static int __set_cpus_allowed_ptr_locked(struct task_struct *p, > > * Kernel threads are allowed on online && !active CPUs > > */ > > cpu_valid_mask = cpu_online_mask; > > + } else if (!cpumask_subset(new_mask, cpu_allowed_mask)) { > > + ret = -EINVAL; > > + goto out; > > So, IIUC, this should make the sched_setaffinity() syscall fail and > return -EINVAL to userspace if it tries to put 64bits CPUs in the > affinity mask of a 32 bits task, which I think makes sense. > > But what about affinity change via cpusets? e.g., if a 32 bit task is > migrated to a cpuset with 64 bit CPUs, then the migration will be > 'successful' and the task will appear to be in the destination cgroup, > but the actual affinity of the task will be something completely > different? Yeah, the cpuset code ignores the return value of set_cpus_allowed_ptr() in update_tasks_cpumask() so the failure won't be propagated, but then again I think that might be the right thing to do. Nothing prevents 32-bit and 64-bit tasks from co-existing in the same cpuseti afaict, so forcing the 64-bit tasks onto the 32-bit-capable cores feels much worse than the approach taken here imo. Nothing says we _have_ to schedule on all of the cores in the mask. The interesting case is what happens if the cpuset for a 32-bit task is changed to contain only the 64-bit-only cores. I think that's a userspace bug, but the fallback rq selection should avert disaster. Will _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel