linux-kernel - Re: [PATCH v5 tip/core/rcu 15/16] rcu: Make RCU-tasks wait for idle tasks

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Message-ID: <20140813144332.GW4752@linux.vnet.ibm.com>
Date:	Wed, 13 Aug 2014 07:43:32 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH v5 tip/core/rcu 15/16] rcu: Make RCU-tasks wait for idle
 tasks

On Wed, Aug 13, 2014 at 10:13:01AM -0400, Steven Rostedt wrote:
> On Wed, 13 Aug 2014 16:07:05 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Wed, Aug 13, 2014 at 09:51:32AM -0400, Steven Rostedt wrote:
> > 
> > > I still need to look at the patches, but if this is just for the idle
> > > case, then we don't need it. The idle case can be solved with a simple
> > > sched_on_each_cpu(). I need a way to solve waiting for processes to
> > > finish from a preemption point.
> > > 
> > > That's all I want, and if we can remove the "idle" case and document it
> > > well that it's not covered and a sched_on_each_cpu() may be needed,
> > > then I'm fine with that.
> > > 
> > > 	sched_on_each_cpu(dummy_op);
> > > 	call_rcu_tasks(free_tramp);
> > 
> > Sure, but why not dtrt and push rcu_idle hooks all the way down into the
> > idle drivers if and where appropriate?
> > 
> > There isn't _that_ much idle driver code. Also, some stuff should be
> > cleaned up; we're already calling stop_critical_timings() in the generic
> > idle code, and then calling it again in the cpuidle drivers.
> 
> True, perhaps the rcu code should hook into the stop_critical_timings
> code?

Is that safe given that stop_critical_timings() is invoked around other things?
Let's see...

o	drivers/acpi/acpi_pad.c, power_saving_thread().

	Looks like a kthread that does idle injection.  Currently, RCU
	sees it as not a quiescent state.  Would it kill these guys to
	put in a comment or two about what this is for???

	So adding rcu_idle_enter() and rcu_idle_exit() here might
	actually fix a bug, though it is not clear how long this thing
	actually runs.  If only for a few milliseconds, no harm done.

o	drivers/acpi/processor_idle.c, acpi_idle_do_entry().

	Hmmm...  Is the idle loop really simpler these days?  Or is the
	complexity just better hidden?  :-/

	So acpi_idle_do_entry() is called from several places, will chase
	them down later.  Does stop_critical_timings() nest?

o	drivers/thermal/intel_powerclamp.c, clamp_thread().

	Looks similar to power_saving_thread(), but for thermal control.
	Probably short term, shouldn't be a problem either way.

o	kernel/printk/printk.c, console_cont_flush().

	call_console_drivers() certainly isn't idle.  Might not use
	RCU read sides, but...

o	kernel/printk/printk.c, console_unlock()

	Same as console_cont_flush().

So the first three look OK to hook rcu_idle_enter() and rcu_idle_exit()
into, but the last two don't look so good.

That said, if you are OK not tracing the stuff under stop_critical_timings(),
then I can use the RCU dyntick-idle state and not wake anything up.

							Thanx, Paul

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