Re: [PATCH tip/core/rcu 2/2] rcu: Check for wakeup-safe conditions in rcu_read_unlock_special()

[Peter Zijlstra]

On Tue, Apr 02, 2019 at 06:18:53AM -0700, Paul E. McKenney wrote:
> On Tue, Apr 02, 2019 at 09:09:53AM +0200, Peter Zijlstra wrote:
> > On Mon, Apr 01, 2019 at 10:22:57AM -0700, Paul E. McKenney wrote:

> > > Or am I missing something that gets the scheduler on the job faster?
> > 
> > Oh urgh, yah. So normally we only twiddle with the need_resched state:
> > 
> >  - while preempt_disabl(), such that preempt_enable() will reschedule
> >  - from interrupt context, such that interrupt return will reschedule
> > 
> > But the usage here 'violates' those rules and then there is an
> > unspecified latency between setting the state and it getting observed,
> > but no longer than 1 tick I would think.
> 
> In general, yes, which is fine (famous last words) for normal grace
> periods but not so good for expedited grace periods.
> 
> > I don't think we can go NOHZ with need_resched set, because the moment
> > we hit the idle loop with that set, we _will_ reschedule.
> 
> Agreed, and I believe that transitioning to usermode execution also
> gives the scheduler a chance to take action.
> 
> The one exception to this is when a nohz_full CPU running in nohz_full
> mode does a system call that decides to execute for a very long time.
> Last I checked, the scheduling-clock interrupt did -not- get retriggered
> in this case, and the delay could be indefinite, as in bad even for
> normal grace periods.

Right, there is that.

> > So in that respect the irq_work suggestion I made would fix things
> > properly.
> 
> But wouldn't the current use of set_tsk_need_resched(current) followed by
> set_preempt_need_resched() work just as well in that case?  The scheduler
> would react to these at the next scheduler-clock interrupt on their
> own, right?  Or am I being scheduler-naive again?

Well, you have that unspecified delay. By forcing the (self) interrupt
you enforce a timely response.