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

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Date:	Thu, 16 May 2013 13:38:12 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Don Zickus <dzickus@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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?

But they are protected inside rcu_nmi_*() functions, that's the only thing we need.
If this interrupt userspace then we resume back to it quickly after the NMI and
re-enter EQS.

No need to restart the tick for that. A remote CPU that wants a quiescent state
from the dyntick CPU will notice soon enough the EQS.

We can certainly drop the perf tick for NMI watchdog:

1) As long as there are no flexible events competing on the CPU, no rotation
should be needed.

2) We don't want event throttling for the watchdog. There is even a hack to
handle that:

       /* Callback function for perf event subsystem */
       static void watchdog_overflow_callback(struct perf_event *event,
                                              struct perf_sample_data *data,
                                              struct pt_regs *regs)
       {
                 /* Ensure the watchdog never gets throttled */
                 event->hw.interrupts = 0;
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