Re: [PATCH tip/core/rcu 02/15] rcu: Use timer as backstop for NOCB deferred wakeups

[Paul E. McKenney]

On Tue, Jul 25, 2017 at 06:17:10PM -0400, Steven Rostedt wrote:
> On Tue, 25 Jul 2017 12:18:14 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Tue, Jul 25, 2017 at 02:12:20PM -0400, Steven Rostedt wrote:
> > > On Mon, 24 Jul 2017 14:44:31 -0700
> > > "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > >   
> > > > The handling of RCU's no-CBs CPUs has a maintenance headache, namely
> > > > that if call_rcu() is invoked with interrupts disabled, the rcuo kthread
> > > > wakeup must be defered to a point where we can be sure that scheduler
> > > > locks are not held.  Of course, there are a lot of code paths leading
> > > > from an interrupts-disabled invocation of call_rcu(), and missing any
> > > > one of these can result in excessive callback-invocation latency, and
> > > > potentially even system hangs.  
> > > 
> > > What about using irq_work? That's what perf and ftrace use for such a
> > > case.  
> > 
> > I hadn't looked at irq_work before, thank you for the pointer!
> > 
> > I nevertheless believe that timers work better in this particular case
> > because they can be cancelled (which appears to be the common case), they
> 
> Is the common case here that it doesn't trigger? That is, the
> del_timer() will be called?

If you have lots of call_rcu() invocations, many of them will be invoked
with interrupts enabled, and a later one with interrupts enabled will
take care of things for the earlier ones.  So there can be workloads
where this is the case.

> > normally are not at all time-critical, and because running in softirq
> > is just fine -- no need to run out of the scheduling-clock interrupt.
> 
> irq_work doesn't always use the scheduling clock. IIRC, it will simply
> trigger a interrupt (if the arch supports it), and the work will be
> done when interrupts are enabled (the interrupt that will do the work
> will trigger)

Ah, OK, so scheduling clock is just the backstop.  Still, softirq
is a bit nicer to manage than hardirq.

> > Seem reasonable?
> 
> Don't know. With irq_work, you just call it and forget about it. No
> need to mod or del timers.

But I could have a series of call_rcu() invocations with interrupts
disabled, so I would need to interact somehow with the irq_work handler.
Either that or dynamically allocate the needed data structure.

Or am I missing something here?

							Thanx, Paul