Messages in this thread | | | Date | Wed, 7 Mar 2012 09:09:39 -0500 | Subject | Re: [GIT PULL] RCU changes for v3.3 | From | Josh Boyer <> |
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On Wed, Mar 7, 2012 at 6:44 AM, Sergey Senozhatsky <sergey.senozhatsky@gmail.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@linux.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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