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

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Date:	Sun, 19 May 2013 09:18:43 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.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 02:32:58PM -0400, Steven Rostedt wrote:
> On Thu, 2013-05-16 at 19:56 +0200, Peter Zijlstra wrote:
> 
> > I suppose the fundamental question was: will receiving NMIs negate NO_HZ_FULL's
> > functionality? That is, will the getting of NMIs make us drop out of NO_HZ_FULL
> > and re-enable all sorts of things?
> 
> It shouldn't. The nmi_enter() notifies RCU that it can no longer ignore
> this CPU, where as nmi_enter() tells it that it can ignore it, as it has
> re-entered user space.
> 
> > 
> > Because clearly RCU needs to exit from EQS, which might (or might not) mean
> > leaving NO_HZ_FULL.
> 
> Yep, but the two are pretty much agnostic from each other.
> 
> We only need to leave NO_HZ_FULL if RCU (or anything for that matter)
> required having a tick again. But as Paul said, getting an NMI in idle
> wont restart the tick, so there's no need to restart it here either.
> 
> Now if an NMI were to do a call_rcu() then it would require a tick. But
> NMIs doing call_rcu() has much bigger issues to worry about ;-)

Someone invoking call_rcu() from an NMI handler will get what they deserve,
good and hard!  ;-)

							Thanx, Paul

> -- Steve
> 
> > 
> > I'm not entirely up-to-date on those details.
> 
> 
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