* Peter Zijlstra peterz@infradead.org [230913 12:13]:
On Wed, Sep 13, 2023 at 10:51:25AM -0400, Liam R. Howlett wrote:
- Peter Zijlstra peterz@infradead.org [230913 09:53]:
On Tue, Sep 12, 2023 at 08:56:47PM -0400, Liam R. Howlett wrote:
diff --git a/init/main.c b/init/main.c index ad920fac325c..f74772acf612 100644 --- a/init/main.c +++ b/init/main.c @@ -696,7 +696,7 @@ noinline void __ref __noreturn rest_init(void) */ rcu_read_lock(); tsk = find_task_by_pid_ns(pid, &init_pid_ns);
- tsk->flags |= PF_NO_SETAFFINITY;
- tsk->flags |= PF_NO_SETAFFINITY | PF_IDLE; set_cpus_allowed_ptr(tsk, cpumask_of(smp_processor_id())); rcu_read_unlock();
Hmm, isn't that pid-1 you're setting PF_IDLE on?
Yes, thanks. I think that is what Geert is hitting with my patch.
debug __might_resched() in kernel/sched/core.c is failing to return in that first (complex) if statement. His report says pid 1 so this is likely the issue.
The task becoming idle is 'current' at this point, see the cpu_startup_entry() call below.
Would not something like so be the right thing?
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 2299a5cfbfb9..802551e0009b 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -9269,7 +9269,7 @@ void __init init_idle(struct task_struct *idle, int cpu) * PF_KTHREAD should already be set at this point; regardless, make it * look like a proper per-CPU kthread. */
- idle->flags |= PF_IDLE | PF_KTHREAD | PF_NO_SETAFFINITY;
- idle->flags |= PF_KTHREAD | PF_NO_SETAFFINITY;
I am concerned this will alter more than just the current task, which would mean more modifications later. There is a comment about it being called 'more than once' and 'per cpu' so I am hesitant to change the function itself.
Although I am unsure of the call path.. fork_idle() -> init_idle() I guess?
There's only 2 ways to get into do_idle(), through cpu_startup_entry() and play_idle_precise(). The latter already frobs PF_IDLE since it is the forced idle path, this then leaves cpu_startup_entry() which is the regular idle path.
All idle threads will end up calling into it, the boot CPU through the rest_init() and the SMP cpus through arch SMP bringup.
IOW, this ensures all idle loops will have PF_IDLE set but not the pre-idle loop setup code these threads run.
Thanks for the information. This does leave the init_idle() function in the odd state of not setting PF_IDLE, but I guess that's okay?