On Wed, Oct 29, 2014 at 05:05:24PM -0700, Paul E. McKenney wrote:
> > > Although it is true that tiny RCU cannot hang a synchronize_rcu()
> > > grace period, it most certainly can hang a call_rcu() grace period
> > > in exactly the same way.
> >
> > Sorry for being a pain in the neck - just want to make sure I am following.
>
> No worries!
>
> > I only see possibility to cause callbacks not being called for "too long"
> > in case a system has lots of nested interrupts and rcu_idle_enter_common()
> > is not being called from hardware interrupt context as result. How could
> > rcu_is_cpu_rrupt_from_idle() help here?
>
> Let's start assuming that something in the idle loop posts a callback,
> and then let me see if I understand your reasoning...
>
> 1. The system is idle and stays that way, no runnable tasks.
>
> 2. An interrupt occurs. Upon return from interrupt, rcu_irq_exit()
> is invoked, which calls rcu_idle_enter_common(), which in turn
> calls rcu_sched_qs(), which does a raise_softirq(RCU_SOFTIRQ).
>
> 3. The softirq happens shortly and invokes rcu_process_callbacks(),
> which invokes __rcu_process_callbacks().
>
> 4. So now callbacks can be invoked. At least they can be if
> ->donetail has been updated. Which it will have been because
> rcu_sched_qs() invokes rcu_qsctr_help().
Yes, that is exactly my reasoning.
> So your point that rcu_is_cpu_rrupt_from_idle() might be redundant could
> well be valid -- sorry for being so dismissive earlier.
>
> > > > > Now, if you -can- get the userspace-execution indication into
> > > > > rcu_irq_exit(), this might be of interest. However, it might be
> > > > > faster
> > > > > to simply let the scheduling-clock interrupt do the job as it
> > > > > currently
> > > > > does, especially for workloads with lots of interrupts.
> > > > >
> > > > > Or did you have something else in mind?
> > > >
> > > > Nope. I would even leave as is tiny RCU's rcu_is_cpu_rrupt_from_idle()
> > > > for clarity then ;)
> > >
> > > Also to avoid userspace execution from preventing RCU callbacks from
> > > ever being invoked. ;-)
> >
> > Hmm.. Am I missing something else? I did not remove the userspace check
> > from the scheduling-clock interrupt:
> >
> > @@ -250,7 +240,7 @@ void rcu_bh_qs(void)
> > void rcu_check_callbacks(int cpu, int user)
> > {
> > RCU_TRACE(check_cpu_stalls());
> > - if (user || rcu_is_cpu_rrupt_from_idle())
> > + if (user)
> > rcu_sched_qs();
> > else if (!in_softirq())
> > rcu_bh_qs();
>
> Probably just me being confused. Hopefully so, as shrinking TINY_RCU
> further will probably be welcome.
Should I resend tiny-only patch?
> Have you done any testing of this change?
Just booted to a unicore kernel and dd'ed 1G of /dev/sda to /dev/null.
> Thanx, Paul
>
--
Regards,
Alexander Gordeev
[email protected]
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