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Subject: Re: [PATCH 3/8] Use process freezer for cpu-hotplug
From: Nigel Cunningham <nigel@nigel.suspend2.net>
Reply-To: nigel@nigel.suspend2.net
To: Nathan Lynch <ntl@pobox.com>
Cc: Ingo Molnar <mingo@elte.hu>, Gautham R Shenoy <ego@in.ibm.com>,
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Date: Sat, 07 Apr 2007 08:22:35 +1000
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Hi.

On Fri, 2007-04-06 at 12:47 -0500, Nathan Lynch wrote:
> Ingo Molnar wrote:
> > 
> > * Nathan Lynch <ntl@pobox.com> wrote:
> > 
> > > > -	raw_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE, hcpu);
> > > > +	if (freeze_processes(FE_HOTPLUG_CPU)) {
> > > > +		thaw_processes(FE_HOTPLUG_CPU);
> > > > +		return -EBUSY;
> > > > +	}
> > > > +
> > > 
> > > If I'm understanding correctly, this will cause
> > > 
> > > # echo 0 > /sys/devices/system/cpu/cpuX/online
> > > 
> > > to sometimes fail, and userspace is expected to try again?  This will 
> > > break existing applications.
> > > 
> > > Perhaps drivers/base/cpu.c:store_online should retry as long as 
> > > cpu_up/down return -EBUSY.  That would avoid a userspace-visible 
> > > interface change.
> > 
> > yeah. I'd even suggest a freeze_processes_nofail() API instead, that 
> > does this internally, without burdening the callsites. (and once the 
> > freezer becomes complete then freeze_processes_nofail() == 
> > freeze_processes())
> 
> Yeah, I just realized that an implementation of my proposal would busy
> loop in the kernel forever if a silly admin tried to offline the last
> cpu (we're already using -EBUSY for that case), so
> freeze_processes_nofail is a better idea :-)

If there's only one online cpu, shouldn't it return -EINVAL?

Regards,

Nigel