linux-kernel - Re: [PATCH] rcu: Remove redundant rcu_is_cpu_rrupt_from

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Date:	Wed, 29 Oct 2014 08:46:15 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Remove redundant rcu_is_cpu_rrupt_from_idle()
 function

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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