linux-kernel - Re: [RFC][PATCH 12/13] stop

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Date:	Tue, 23 Jun 2015 13:20:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	paulmck@...ux.vnet.ibm.com, tj@...nel.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org, der.herr@...r.at, dave@...olabs.net,
	riel@...hat.com, viro@...IV.linux.org.uk,
	torvalds@...ux-foundation.org
Subject: Re: [RFC][PATCH 12/13] stop_machine: Remove lglock

On Tue, Jun 23, 2015 at 12:55:48PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 23, 2015 at 12:09:32PM +0200, Peter Zijlstra wrote:
> > We can of course slap a percpu-rwsem in, but I wonder if there's
> > anything smarter we can do here.
> 
> Urgh, we cannot use percpu-rwsem here, because that would require
> percpu_down_write_trylock(), and I'm not sure we can get around the
> sync_sched() for that.
> 
> Now try_stop_cpus(), which requires the down_write_trylock() is used to
> implement synchronize_sched_expedited().
> 
> Using sync_sched() to implement sync_sched_expedited would make me
> happy, but it does somewhat defeat the purpose.

Paul, why does this use stop_machine anyway? I seemed to remember you
sending resched IPIs around.

The rcu_sched_qs() thing would set passed_quiesce, which you can then
collect to gauge progress.

Shooting IPIs around is bad enough, but running a full blown
stop_machine is really blunt and heavy.


Also, OMFG @ 74b51ee152b6 ("ACPI / osl: speedup grace period in
acpi_os_map_cleanup"), that's an expedited use to help the nVidiot
binary blob. WTF!!

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