linux-kernel - Re: [PATCH 2/4] stop_machine: reimplement using cpuhog

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Message-ID: <20100308193737.GA2466@osiris.boeblingen.de.ibm.com>
Date:	Mon, 8 Mar 2010 20:37:37 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	rusty@...tcorp.com.au, sivanich@....com,
	torvalds@...ux-foundation.org, mingo@...e.hu, peterz@...radead.org,
	dipankar@...ibm.com, josh@...edesktop.org,
	paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 2/4] stop_machine: reimplement using cpuhog

On Mon, Mar 08, 2010 at 07:27:08PM +0100, Oleg Nesterov wrote:
> On 03/08, Heiko Carstens wrote:
> >
> > On Tue, Mar 09, 2010 at 12:53:21AM +0900, Tejun Heo wrote:
> > >
> > > diff --git a/arch/s390/kernel/time.c b/arch/s390/kernel/time.c
> > > index 65065ac..afe429e 100644
> > > --- a/arch/s390/kernel/time.c
> > > +++ b/arch/s390/kernel/time.c
> > > @@ -397,7 +397,6 @@ static void __init time_init_wq(void)
> > >  	if (time_sync_wq)
> > >  		return;
> > >  	time_sync_wq = create_singlethread_workqueue("timesync");
> > > -	stop_machine_create();
> > >  }
> > >
> > >  /*
> >
> > The reason we introduced stop_machine_create/destroy was to have a non-failing
> > variant that doesn't rely on I/O.
> > If we ever see a timesync machine check no I/O will succeed (it blocks) until
> > clocks have been synchronized. That means also that we rely on the non-blocking
> > semantics that those functions must have that are called via stop_machine.
> > This isn't true anymore with the cpu hog infrastructure:
> > if passed a blocking function that could wait on I/O we won't see any progress
> > anymore and the machine is dead.
> 
> Could you please spell?
> 
> How cpuhog can make a difference? Afaics, we shouldn't pass a blocking callback
> to hog_cpus/hog_one_cpu.

Well, it might me true that this shouldn't be done. But I don't see a reason why
in general it wouldn't work to pass a function that would block.
So it's just a matter of time until somebody uses it for such a purpose.
For the current stop_machine implementation it would be broken to pass a blocking
function (preemption disabled, interrupts disabled).
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