Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about disabling preemption

[Joel Fernandes]

On Thu, Oct 18, 2018 at 10:52:23PM -0400, Steven Rostedt wrote:
> On Thu, 18 Oct 2018 19:25:29 -0700
> Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> 
> > On Thu, Oct 18, 2018 at 09:50:35PM -0400, Steven Rostedt wrote:
> > > On Thu, 18 Oct 2018 18:26:45 -0700
> > > Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> > >   
> > > > Yes, local_irq_restore is light weight, and does not check for reschedules.
> > > > 
> > > > I was thinking of case where ksoftirqd is woken up, but does not run unless
> > > > we set the NEED_RESCHED flag. But that should get set anyway since probably
> > > > ksoftirqd is of high enough priority than the currently running task..
> > > > 
> > > > Roughly speaking the scenario could be something like:
> > > > 
> > > > rcu_read_lock();
> > > >                  <-- IPI comes in for the expedited GP, sets exp_hint
> > > > local_irq_disable();
> > > > // do a bunch of stuff
> > > > rcu_read_unlock();   <-- This calls the rcu_read_unlock_special which raises
> > > >                          the soft irq, and wakesup softirqd.  
> > > 
> > > If softirqd is of higher priority than the current running task, then
> > > the try_to_wake_up() will set NEED_RESCHED of the current task here.
> > >   
> > 
> > Yes, only *if*. On my system, ksoftirqd is CFS nice 0. I thought expedited
> > grace periods are quite important and they should complete quickly which is
> > the whole reason for interrupting rcu read sections with an IPI and stuff.
> > IMO there should be no harm in setting NEED_RESCHED unconditionally anyway
> > for possible benefit of systems where the ksoftirqd is not of higher priority
> > than the currently running task, and we need to run it soon on the CPU. But
> > I'm Ok with whatever Paul and you want to do here.
> 
> 
> Setting NEED_RESCHED unconditionally wont help. Because even if we call
> schedule() ksoftirqd will not be scheduled! If it's CFS nice 0, and the
> current task still has quota to run, if you call schedule, you'll just
> waste time calculating that the current task should still be running.
> It's equivalent to calling yield() (which is why we removed all yield()
> users in the kernel, because *all* of them were buggy!). This is *why*
> it only calls schedule *if* softirqd is of higher priority.

Yes, ok. you are right the TTWU path should handle setting the NEED_RESCHED
flag or not and unconditionally setting it does not get us anything. I had to
go through the code a bit since it has been a while since I explored it.

So Paul, I'm Ok with your latest patch for the issue we discussed and don't
think much more can be done barring raising of ksofitrqd priorities :-) So I
guess the synchronize_rcu_expedited will just cope with the deal between
local_irq_enable and the next scheduling point.. :-)

thanks,

- Joel