linux-kernel - Re: [PATCH] watchdog: touch_nmi_watchdog should only touch local cpu not every one

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Message-Id: <20101105125855.20e5ce4c.akpm@linux-foundation.org>
Date:	Fri, 5 Nov 2010 12:58:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Don Zickus <dzickus@...hat.com>, fweisbec@...il.com,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] watchdog:  touch_nmi_watchdog should only touch local
 cpu not every one

On Fri, 5 Nov 2010 15:51:18 +0200
Sergey Senozhatsky <sergey.senozhatsky@...il.com> wrote:

> On (11/04/10 21:18), Don Zickus wrote:
> >  void touch_nmi_watchdog(void)
> >  {
> > +	/*
> > +	 * Using __raw here because some code paths have
> > +	 * preemption enabled.  If preemption is enabled
> > +	 * then interrupts should be enabled too, in which
> > +	 * case we shouldn't have to worry about the watchdog
> > +	 * going off.
> > +	 */
> > +	__raw_get_cpu_var(watchdog_nmi_touch) = true;
> > +
> > +	touch_softlockup_watchdog();
> > +}
> > +EXPORT_SYMBOL(touch_nmi_watchdog);
> > +
> > +void touch_all_nmi_watchdogs(void)
> > +{
> >  	if (watchdog_enabled) {
> >  		unsigned cpu;
> >  
> > @@ -151,7 +166,7 @@ void touch_nmi_watchdog(void)
> >  	}
> >  	touch_softlockup_watchdog();
> >  }
> > -EXPORT_SYMBOL(touch_nmi_watchdog);
> > +EXPORT_SYMBOL(touch_all_nmi_watchdogs);
> >  
> 
> Hello,
> Seems like no one is actually calling touch_all_nmi_watchdogs, as for now. 
> Right?

Yes, there doesn't seem a lot of point in adding the interface unless
we have callers.

> 
> Minor nit
> 
> 	touch_all_nmi_watchdogs:
> 	...
> 	for_each_present_cpu(cpu) {
> 		if (per_cpu(watchdog_nmi_touch, cpu) != true)
> 			per_cpu(watchdog_nmi_touch, cpu) = true;
> 	}
> 
> 
> which is, I belive, could be simplified to 
> 	for_each_present_cpu(cpu) {
> 		per_cpu(watchdog_nmi_touch, cpu) = true;
> 	}

We sometimes do this trick to avoid dirtying lots of cachelines which
already held the correct value.  It'll be extra-benefical when dealing
with other CPU's data, I expect.
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