linux-kernel - Re: [GIT PULL] RCU changes for v3.3

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Date:	Wed, 7 Mar 2012 07:27:03 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Boyer <jwboyer@...il.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, Andre@...per.es
Subject: Re: [GIT PULL] RCU changes for v3.3

On Wed, Mar 07, 2012 at 09:09:39AM -0500, Josh Boyer wrote:
> On Wed, Mar 7, 2012 at 6:44 AM, Sergey Senozhatsky
> <sergey.senozhatsky@...il.com> wrote:
> > > > > ------------------------------------------------------------------------
> >> > > >
> >> > > > idle: Avoid using RCU when RCU thinks the CPU is idle
> >> > > >
> >> > > > The x86 idle loops invoke cpuidle_idle_call() which uses tracing
> >> > > > which uses RCU.  Unfortunately, these idle loops have already
> >> > > > told RCU to ignore this CPU when they call it.  This patch hacks
> >> > > > the idle loops to avoid this problem, but probably causing several
> >> > > > other problems in the process.
> >> > > >
> >> > > > Not-yet-signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> >> > > > ---
> >> > >
> >> > > Hi Paul
> >> > >
> >> > > Just tested it on my x86_64 machine, but warnings are still here
> >> > >
> >> > > Thanks !
> >> >
> >> > Gah!!!  The mwait_idle() function itself (which is the default value of
> >> > the pm_idle function pointer) uses tracing and thus RCU!  What part of
> >> > "don't use RCU from idle CPUs" was unclear, one wonders?
> >> >
> >> > Ah well, the good news is that we can now detect such abuse and fix it.
> >> >
> >> > But fixing it appears to require pushing rcu_idle_enter() and
> >> > rcu_idle_exit() pairs down to the bottom of each and every idle loop
> >> > and governor.
> >> >
> >> > So...  The cpuidle_idle_call() function has an idle loop inside of itself,
> >> > namely the ->enter() call for the desired target state.  It does tracing
> >> > on both sides of that call.  Should the ->enter() calls actually avoid
> >> > use of tracing, I could push the rcu_idle_enter() and rcu_idle_exit()
> >> > down into cpuidle_idle_call().  We seem to have a ladder_governor and
> >> > a menu_governor in 3.2, and these have states, which in turn have ->enter
> >> > functions.
> >> >
> >> > Hmmm...  Residual power dissipation is given in milliwatts.  I could
> >> > imagine some heartburn from many of the more aggressive embedded folks,
> >> > given that they might prefer microwatts -- or maybe even nanowatts,
> >> > for all I know.
> >> >
> >> > There are a bunch of states defined in drivers/idle/intel_idle.c,
> >> > and these use intel_idle() as their ->enter() states.  This one looks
> >> > to have a nice place for rcu_idle_enter() and rcu_idle_exit().
> >> >
> >> > But I also need to push rcu_idle_enter() and rcu_idle_exit() into any
> >> > function that can be assigned to pm_idle():  default_idle(), poll_idle(),
> >> > mwait_idle(), and amd_e400_idle().  OK, that is not all -that- bad,
> >> > though this must also be done for a number of other architectures as well.
> >> >
> >> > OK, will post a patch.  I will need testing -- clearly my testing on KVM
> >> > is missing a few important code paths...
> >>
> >> And here is another version of the patch.
> > Hello,
> > I just hit the same problem.
> >
> > Is this patch scheduled for 3.3 until release or will land during 3.4
> > merge window?
> 
> There are 3 patches in Paul's 3.4 queue, and another 9 of Steven Rostedt's
> that fix this.  They'll wind up in 3.4.
> 
> I looked at reverting the patch that adds this new warning and it doesn't
> trivially revert.

What are you based on?  I should be able to disable the warning for
you easily enough.

							Thanx, Paul

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