linux-kernel - Re: [RFC PATCH 0/6] stop_machine: kill stop_cpus_mutex and stop_cpus

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Message-ID: <20150626023149.GA6178@redhat.com>
Date:	Fri, 26 Jun 2015 04:31:49 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>, paulmck@...ux.vnet.ibm.com,
	tj@...nel.org, mingo@...hat.com, der.herr@...r.at,
	dave@...olabs.net, riel@...hat.com, viro@...IV.linux.org.uk,
	torvalds@...ux-foundation.org
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/6] stop_machine: kill stop_cpus_mutex and
	stop_cpus_lock

As always, forgot to mention...

Also. We can turn these stop_work_alloc/stop_work_free into the generic
helpers which (I think) can have more users.

On 06/26, Oleg Nesterov wrote:
>
> On 06/25, Peter Zijlstra wrote:
> >
> > On Tue, Jun 23, 2015 at 07:24:16PM +0200, Oleg Nesterov wrote:
> > >
> > > 	lock_stop_cpus_works(cpumask)
> > > 	{
> > > 		for_each_cpu(cpu, cpumask)
> > > 			mutex_lock(per_cpu(cpu_stopper_task, cpu).work_mutex);
> > > 	}
> > >
> > > 	unlock_stop_cpus_works(cpumask)
> > > 	{
> > > 		for_each_cpu(cpu, cpumask)
> > > 			mutex_lock(...);
> > > 	}
> > >
> > > which should be used instead of stop_cpus_mutex. After this change
> > > stop_two_cpus() can just use stop_cpus().
> >
> > Right, lockdep annotating that will be 'interesting' though.
>
> Sure, and this is too inefficient, this is only to explain what
> I mean.
>
> How about this series? Untested. For review.
>
> > And
> > stop_two_cpus() then has the problem of allocating a cpumask.
>
> Yes, but we can avoid this, see the changelog in 5/6.
>
> > Simpler to
> > let it keep 'abuse' the queueing spinlock in there.
>
> Not sure.
>
> And note that this series kills stop_cpus_mutex, so that multiple
> stop_cpus()'s / stop_machine()'s can run in parallel if cpumask's
> do not overlap.
>
> Note also the changelog in 6/6, we can simplify + optimize this code
> a bit more.
>
> What do you think?
>
> Oleg.
>
>  include/linux/lglock.h  |    5 -
>  kernel/locking/lglock.c |   22 -----
>  kernel/stop_machine.c   |  197 ++++++++++++++++++++++++++++-------------------
>  3 files changed, 119 insertions(+), 105 deletions(-)

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