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

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Message-ID: <20140813182406.GE4752@linux.vnet.ibm.com>
Date:	Wed, 13 Aug 2014 11:24:07 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.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 09:43:05AM -0700, Jacob Pan wrote:
> On Wed, 13 Aug 2014 18:30:15 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Wed, Aug 13, 2014 at 07:43:32AM -0700, Paul E. McKenney wrote:
> > > 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/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.
> > > 
> > 
> > There's patches somewhere that make that go-away
> > 
> >   https://lkml.org/lkml/2014/6/4/56
> > 
> > Jacob was going to look at that.
> 
> yes, it is on my plate, plan to submit for 3.18. the idle period is
> only for a few miliseconds, default to 6ms.

OK, 6ms shouldn't be a problem.  But if you want much longer, you
will need to do rcu_idle_enter() and rcu_idle_exit().  Of course, if
you are getting rid of this entirely, no need to worry -- though the
patch looks like it just uses a different idle loop.

							Thanx, Paul

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