linux-kernel - Re: [RFC][PATCH][EXPERIMENTAL] CPU hotplug with frozen tasks

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Message-Id: <200704162306.06910.rjw@sisk.pl>
Date:	Mon, 16 Apr 2007 23:06:06 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Oleg Nesterov <oleg@...sign.ru>
Subject: Re: [RFC][PATCH][EXPERIMENTAL] CPU hotplug with frozen tasks

On Monday, 16 April 2007 09:05, Pavel Machek wrote:
> Hi!
> 
> > As I said before, we have a problem with using the CPU hotplug for suspending
> > because of the notifiers that are called from within cpu_up()/cpu_down() and
> > (sometimes) assume that the system is fully functional.
> > 
> > One obvious solution of this problem would be to make the notifiers behave
> > differently if tasks are frozen, but for this purpose we'd need to tell them
> > that this is the case.  In principle, we could do it in many different ways
> > (eg. by using a global variable, with the help of suspend notifiers etc.), but
> > IMO one of the cleanest methods woud be to use some special values for the
> > notifications occuring while tasks are frozen (eg. CPU_DEAD_FROZEN instead of
> > CPU_DEAD etc.).  In that case the notifiers could react in some special ways
> > to the "FROZEN" notfifications and that would allow us to simplify some code
> > paths (eg. in the microcode driver).
> > 
> > The appended patch introduces such "FROZEN" notfifications, modifies the CPU
> > hotplug core to use them and updates all of the users of CPU hotplug notifiers
> > to recognize them.  For now, they are treated in the same way as the
> > corresponding "normal" notifications, but I'm going to modify the microcode
> > driver to really use them and I believe that some other subsystems can benefit
> > from using them as well.
> > 
> > The patch is totally experimental and untested, although it's been successfully
> > compiled on x86_64 and it's main purpose is to show what exactly I
> > mean. :-)
> 
> Looks sane to me.
> 
> > Index: linux-2.6.21-rc6/kernel/cpu.c
> > ===================================================================
> > --- linux-2.6.21-rc6.orig/kernel/cpu.c	2007-04-16 00:24:56.000000000 +0200
> > +++ linux-2.6.21-rc6/kernel/cpu.c	2007-04-16 00:25:14.000000000 +0200
> > @@ -120,11 +120,12 @@ static int take_cpu_down(void *unused)
> >  }
> >  
> >  /* Requires cpu_add_remove_lock to be held */
> > -static int _cpu_down(unsigned int cpu)
> > +static int _cpu_down(unsigned int cpu, int tasks_frozen)
> >  {
> >  	int err;
> >  	struct task_struct *p;
> >  	cpumask_t old_allowed, tmp;
> > +	unsigned long mod = tasks_frozen ? 0x0008 : 0;
> >  
> 
> Can we get constant instead of 0x0008 here?

Sure.  Updated patch is in the reply to Gautham.

Greetings,
Rafael
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