Commit ac687e6e authored by Peter Zijlstra's avatar Peter Zijlstra

kthread: Extract KTHREAD_IS_PER_CPU

There is a need to distinguish geniune per-cpu kthreads from kthreads
that happen to have a single CPU affinity.

Geniune per-cpu kthreads are kthreads that are CPU affine for
correctness, these will obviously have PF_KTHREAD set, but must also
have PF_NO_SETAFFINITY set, lest userspace modify their affinity and
ruins things.

However, these two things are not sufficient, PF_NO_SETAFFINITY is
also set on other tasks that have their affinities controlled through
other means, like for instance workqueues.

Therefore another bit is needed; it turns out kthread_create_per_cpu()
already has such a bit: KTHREAD_IS_PER_CPU, which is used to make
kthread_park()/kthread_unpark() work correctly.

Expose this flag and remove the implicit setting of it from
kthread_create_on_cpu(); the io_uring usage of it seems dubious at
best.
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarValentin Schneider <valentin.schneider@arm.com>
Tested-by: default avatarValentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210121103506.557620262@infradead.org
parent 22f667c9
...@@ -33,6 +33,9 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data), ...@@ -33,6 +33,9 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
unsigned int cpu, unsigned int cpu,
const char *namefmt); const char *namefmt);
void kthread_set_per_cpu(struct task_struct *k, int cpu);
bool kthread_is_per_cpu(struct task_struct *k);
/** /**
* kthread_run - create and wake a thread. * kthread_run - create and wake a thread.
* @threadfn: the function to run until signal_pending(current). * @threadfn: the function to run until signal_pending(current).
......
...@@ -493,11 +493,36 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data), ...@@ -493,11 +493,36 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
return p; return p;
kthread_bind(p, cpu); kthread_bind(p, cpu);
/* CPU hotplug need to bind once again when unparking the thread. */ /* CPU hotplug need to bind once again when unparking the thread. */
set_bit(KTHREAD_IS_PER_CPU, &to_kthread(p)->flags);
to_kthread(p)->cpu = cpu; to_kthread(p)->cpu = cpu;
return p; return p;
} }
void kthread_set_per_cpu(struct task_struct *k, int cpu)
{
struct kthread *kthread = to_kthread(k);
if (!kthread)
return;
WARN_ON_ONCE(!(k->flags & PF_NO_SETAFFINITY));
if (cpu < 0) {
clear_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
return;
}
kthread->cpu = cpu;
set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
}
bool kthread_is_per_cpu(struct task_struct *k)
{
struct kthread *kthread = to_kthread(k);
if (!kthread)
return false;
return test_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
}
/** /**
* kthread_unpark - unpark a thread created by kthread_create(). * kthread_unpark - unpark a thread created by kthread_create().
* @k: thread created by kthread_create(). * @k: thread created by kthread_create().
......
...@@ -188,6 +188,7 @@ __smpboot_create_thread(struct smp_hotplug_thread *ht, unsigned int cpu) ...@@ -188,6 +188,7 @@ __smpboot_create_thread(struct smp_hotplug_thread *ht, unsigned int cpu)
kfree(td); kfree(td);
return PTR_ERR(tsk); return PTR_ERR(tsk);
} }
kthread_set_per_cpu(tsk, cpu);
/* /*
* Park the thread so that it could start right on the CPU * Park the thread so that it could start right on the CPU
* when it is available. * when it is available.
......
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