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

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Message-ID: <20100308182708.GA8504@redhat.com>
Date:	Mon, 8 Mar 2010 19:27:08 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Heiko Carstens <heiko.carstens@...ibm.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 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.

Oleg.

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