Messages in this thread | | | Date | Wed, 29 Oct 2014 08:46:15 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] rcu: Remove redundant rcu_is_cpu_rrupt_from_idle() function |
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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.
> > 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. ;-)
Thanx, Paul
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