Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT_NONE

[Oleg Nesterov]

On 09/12, Peter Zijlstra wrote:
>
> On Mon, Sep 12, 2016 at 03:05:38PM +0200, Oleg Nesterov wrote:
>
> > But this leads to the question which I wanted to ask many times.
> >
> > Why cond_resched() is not NOP if CONFIG_PREEMPT=y ?
>
> Dunno, nobody bothered to do it? We should keep the might_sleep() of
> course, but the preemption check is pointless.

Yes, agreed, I actually meant _cond_resched().

> > Perhaps we have some users like, just for example,
> > 
> > 	preempt_enable_no_resched();
> > 	cond_resched();
> > 
> > which actually want the should_resched() check even if CONFIG_PREEMPT,
> > but most callers do not?
>
> I would hope not, the few preempt_enable_no_resched() users _should_
> have an actual schedule() call in the _very_ near future.

Me too, and I failed to find something which could be broken... So
perhaps should make it nop and investigate the new bug reports after
that.


Hmm. And  preempt_enable_no_resched_notrace() under TASK_DEAD in
__schedule() should be removed it seems, do_exit() can call __schedule()
directly.

Oleg.