On Wed, Oct 29, 2014 at 08:46:15AM -0700, Paul E. McKenney wrote:
> On Wed, Oct 29, 2014 at 03:16:23PM +0000, Alexander Gordeev wrote:
> > On Wed, Oct 29, 2014 at 07:34:39AM -0700, Paul E. McKenney wrote:
> > > On Wed, Oct 29, 2014 at 01:45:19PM +0000, Alexander Gordeev wrote:
> > > > On Wed, Oct 29, 2014 at 05:47:29AM -0700, Paul E. McKenney wrote:
> > > > > On Wed, Oct 29, 2014 at 12:07:07PM +0100, Alexander Gordeev wrote:
> > > > > > Function rcu_is_cpu_rrupt_from_idle() is called from scheduling-
> > > > > > clock interrupt handler to check if the current CPU was interrupted
> > > > > > from idle. If true, it results in invocation of RCU callbacks. But
> > > > > > the common hardware interrupt exit path also contains similar check
> > > > > > and therefore the call to rcu_is_cpu_rrupt_from_idle() is redundant.
> > > > >
> > > > > By common hardware interrupt exit path, you are meaning the calls
> > > > > to rcu_irq_exit()? If not, please let me know exactly what you
> > > > > mean here.
> > > >
> > > > Yes, I mean rcu_irq_exit().
> > >
> > > Unless you can get the indication of whether or not the original interrupt
> > > came from userspace execution into rcu_irq_exit(), this will not work.
> > > It will result in grace-period hangs on some configurations.
> >
> > Okay, that was my concern wrt tree RCU. By contrast, tiny RCU does not seem
> > able to hang a grace-period, isn't it?
>
> 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.
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?
> > > 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();
Thanks!
> Thanx, Paul
--
Regards,
Alexander Gordeev
agord...@redhat.com
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