Messages in this thread Patch in this message | | | Subject | Re: [PATCH 0/9] kthread detection cleanup | From | Valentin Schneider <> | Date | Wed, 14 Aug 2019 14:43:17 +0100 |
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On 14/08/2019 11:41, Mark Rutland wrote: > A reasonable amount of kernel code looks at task_struct::mm to determine > whether a thread is a kthread or a real user task. This isn't quite right, > since kthreads can have a non-NULL mm when calling use_mm(). > > The correct way to check whether a task is a kthread is to check whether > PF_KTHREAD is set in task_struct::flags, but doing so is a bit unwieldy. > > To make this a bit nicer, this series adds a new is_kthread(tsk) helper, > converts existing code to make use of it, and fixes up a number of erroneous > checks of current->mm. Hopefully this will push people to use the right > approach in future. > > I'm sure there are other instances in the kernel tree where we don't check this > correctly. In this series I'm just trying to fix the instances I'm reasonably > confident are incorrect. >
I've only found one extra location in kernel/sched/ that seems like it wants to use is_kthread(), included below. I can send it out separately after this gets merged.
-----8<----- diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 4c4a0494c39b..0956972f6ea7 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1964,11 +1964,10 @@ static int select_fallback_rq(int cpu, struct task_struct *p) out: if (state != cpuset) { /* - * Don't tell them about moving exiting tasks or - * kernel threads (both mm NULL), since they never - * leave kernel. + * Don't tell them about moving exiting tasks (NULL mm) or + * kernel threads since they never leave kernel. */ - if (p->mm && printk_ratelimit()) { + if ((!is_kthread(p) && p->mm) && printk_ratelimit()) { printk_deferred("process %d (%s) no longer affine to cpu%d\n", task_pid_nr(p), p->comm, cpu); } ----->8----- Regarding sched/fair.c, I am quite convinced all of the p->mm checks are genuine (as in they don't care if it's a kthread or not) - those are all about NUMA balancing. Maybe Rik/Mel can correct me if I'm wrong.
Cheers, Valentin
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