[SRU Hirsute 1/1] sched: Stop PF_NO_SETAFFINITY from being inherited by various init system threads
Thadeu Lima de Souza Cascardo
cascardo at canonical.com
Mon Aug 16 17:40:33 UTC 2021
From: Frederic Weisbecker <frederic at kernel.org>
00b89fe0197f ("sched: Make the idle task quack like a per-CPU kthread")
... added PF_KTHREAD | PF_NO_SETAFFINITY to the idle kernel threads.
Unfortunately these properties are inherited to the init/0 children
through kernel_thread() calls: init/1 and kthreadd. There are several
side effects to that:
1) kthreadd affinity can not be reset anymore from userspace. Also
PF_NO_SETAFFINITY propagates to all kthreadd children, including
the unbound kthreads Therefore it's not possible anymore to overwrite
the affinity of any of them. Here is an example of warning reported
WARNING: CPU: 0 PID: 116 at kernel/rcu/tree_nocb.h:1306 rcu_bind_current_to_nocb+0x31/0x40
2) init/1 does an exec() in the end which clears both
PF_KTHREAD and PF_NO_SETAFFINITY so we are fine once kernel_init()
escapes to userspace. But until then, no initcall or init code can
successfully call sched_setaffinity() to init/1.
Also PF_KTHREAD looks legit on init/1 before it calls exec() but
we better be careful with unknown introduced side effects.
One way to solve the PF_NO_SETAFFINITY issue is to not inherit this flag
on copy_process() at all. The cases where it matters are:
* fork_idle(): explicitly set the flag already.
* fork() syscalls: userspace tasks that shouldn't be concerned by that.
* create_io_thread(): the callers explicitly attribute the flag to the
newly created tasks.
- Fix the issues on init/1 and kthreadd
- Fix the issues on kthreadd children.
- Usermode helper created by an unbound workqueue. This shouldn't
matter. In the worst case it gives more control to userspace
on setting affinity to these short living tasks although this can
be tuned with inherited unbound workqueues affinity already.
Fixes: 00b89fe0197f ("sched: Make the idle task quack like a per-CPU kthread")
Reported-by: Paul E. McKenney <paulmck at kernel.org>
Signed-off-by: Frederic Weisbecker <frederic at kernel.org>
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Tested-by: Paul E. McKenney <paulmck at kernel.org>
(cherry picked from commit a8ea6fc9b089156d9230bfeef964dd9be101a4a9)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
kernel/fork.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/fork.c b/kernel/fork.c
index da2f7d9a370f..183863e6ea85 100644
@@ -2002,7 +2002,7 @@ static __latent_entropy struct task_struct *copy_process(
delayacct_tsk_init(p); /* Must remain after dup_task_struct() */
- p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER | PF_IDLE);
+ p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER | PF_IDLE | PF_NO_SETAFFINITY);
p->flags |= PF_FORKNOEXEC;
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