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

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Message-ID: <20141030175435.GC13273@agordeev.usersys.redhat.com>
Date:	Thu, 30 Oct 2014 17:54:35 +0000
From:	Alexander Gordeev <agordeev@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 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
agordeev@...hat.com
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