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

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Message-ID: <CA+5PVA4CdtQYbJBvpoAa7iyWegBpJaDBcFfXpXYX=pARw_SBSA@mail.gmail.com>
Date:	Wed, 7 Mar 2012 09:09:39 -0500
From:	Josh Boyer <jwboyer@...il.com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 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.

josh
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