Messages in this thread | | | Date | Tue, 6 Sep 2016 17:22:54 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] sched/core: simpler function for sched_exec migration |
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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.
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