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Message-ID: <20130519161843.GI4006@linux.vnet.ibm.com>
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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