Re: [PATCH 3/3] sched: Rework check_for_tasks()

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jun 23, 2014 at 02:52:18PM +0400, Kirill Tkhai wrote:
> > Then again, I suppose anything without rq->lock can and will miss tasks.
> 
> If we use rq->lock it's possible to move check_for_tasks() to kernel/sched/core.c.
> 
> And we can leave TASK_RUNNING check for waking tasks. Maybe something like this?
> 
> static inline void check_for_tasks(int dead_cpu)
> {
> 	struct task_struct *g, *p;
> 	struct rq *rq = cpu_rq(dead_cpu);
> 
> 	read_lock_irq(&tasklist_lock);
> 	raw_spin_lock(&rq->lock)
> 
> 	do_each_thread(g, p) {
> 		if (!p->on_rq && p->state != TASK_RUNNING)
> 			continue;
> 		if (task_cpu(p) != dead_cpu)
> 			continue;
> 
> 		pr_warn("Task %s (pid=%d) is on cpu %d (state=%ld, flags=%x)\n",
> 			p->comm, task_pid_nr(p), dead_cpu, p->state, p->flags);
> 	} while_each_thread(g, p);
> 
> 	raw_spin_unlock(&rq->lock)
> 	read_unlock_irq(&tasklist_lock);
> }
> 
> It still does not give a 100% guarantee... Should we take p->pi_lock for every task?

seeing how rq->lock nests inside of ->pi_lock that's going to be
somewhat icky.

I think we can live with a false negative, given how much people run
this nonsense it'll trigger eventually.

False positives would be bad though :-)

So I think we can keep your original (lock-free) proposal.