On 09/06, chengchao wrote:
>
> the key point is for CONFIG_PREEMPT_NONE=y,
> ...
> it is too much overhead for one task(fork()+exec()), isn't it?
Yes, yes, I see, this is suboptimal. Not sure we actually do care,
but yes, perhaps another helper which migrates the current task makes
sense, I dunno.
But,
> > stop_one_cpu_sync() assumes that cpu == smp_processor_id/task_cpu(current),
> > and thus the stopper thread should preempt us at least after schedule()
> > (if CONFIG_PREEMPT_NONE), so we do not need to synchronize.
> >
> yes. the stop_one_cpu_sync is not a good name, stop_one_cpu_schedule is
> better?
> there is nothing about synchronization.
We need to synchronize with the stopper to ensure it can't touch
cpu_stop_work on stack after stop_one_cpu_sync() returns, and
> > But this is not necessarily true? This task can migrate to another CPU
> > before cpu_stop_queue_work() ?
> >
> before sched_exec() calls stop_one_cpu()/cpu_stop_queue_work(), this
> task(current) cannot migrate to another cpu,because this task is running
> on the cpu.
Why? The running task can migrate to another CPU at any moment. Unless it
runs with preemption disabled or CONFIG_PREEMPT_NONE=y.
And this means that cpu_stop_queue_work() can queue the work on another
CPU != smp_processor_id(), and in this case the kernel can crash because
the pending cpu_stop_work can be overwritten right after return.
So you need something like
void stop_one_cpu_sync(cpu_stop_fn_t fn, void *arg)
{
struct cpu_stop_work work = { .fn = fn, .arg = arg, .done =
NULL };
preempt_disable();
cpu_stop_queue_work(raw_smp_processor_id(), &work);
preempt_enable_no_resched();
schedule();
}
or I am totally confused. Note that it doesn't (and shouldn't) have
the "int cpu" argument.
Oleg.
