linux-kernel - Re: [PATCH 1/2] nohz: Disable LOCKUP_DETECTOR when NO_HZ

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Date:	Thu, 16 May 2013 08:07:06 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Don Zickus <dzickus@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/2] nohz: Disable LOCKUP_DETECTOR when NO_HZ_FULL is
 enabled

On Thu, May 16, 2013 at 10:10:27AM +0200, Peter Zijlstra wrote:
> On Wed, May 15, 2013 at 01:04:01PM -0400, Steven Rostedt wrote:
> > On Wed, 2013-05-15 at 18:59 +0200, Peter Zijlstra wrote:
> > 
> > > At which point we could run the watchdog without perf_event_task_tick().
> > 
> > At which point we can drop the disable LOCKUP_DETECTOR when NO_HZ_FULL
> > is enabled ;-)
> > 
> 
> Can we? The thing I'm worried about is RCU (of course!). ISTR we rely on RCU
> working in NMI context. AFAIR for RCU to work, we need to come out of out magic
> NO_HZ state since that would've put RCU into EQS.
> 
> Frederic, PaulMck?

Not sure I understand the question, but hopefully the verbiage below helps.

Only RCU read-side critical sections need to work in NMI context,
and RCU hooks into nmi_enter() and nmi_exit() to handle this, and this
will work in NO_HZ_FULL in the same way that it works for NO_HZ_IDLE.

But if there are no NMIs, RCU doesn't care.  In other words, RCU needs
to know about NMIs so that it can deal with any RCU read-side critical
sections in the NMI handlers, but RCU doesn't rely on NMIs happening at
any particular time or frequency.

							Thanx, Paul

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