linux-kernel - Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace periods

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Date:	Sat, 21 Feb 2015 09:59:45 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace
 periods

On Sat, Feb 21, 2015 at 05:11:30PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 20, 2015 at 10:38:49AM -0800, Paul E. McKenney wrote:
> > On Fri, Feb 20, 2015 at 06:43:59PM +0100, Peter Zijlstra wrote:
> > > On Fri, Feb 20, 2015 at 09:32:39AM -0800, Arjan van de Ven wrote:
> > > > there's a few others as well that I'm chasing down...
> > > > .. but the flip side, prior to running ring 3 code, why NOT do fast expedites?
> > > 
> > > So my objections are twofold:
> > > 
> > >  - I object to fast expedites in principle; they spray IPIs across the
> > >    system, so ideally we'd not have them at all, therefore also not at
> > >    boot.
> > 
> > There are only a few uses of expedited grace periods, despite their
> > having been in the kernel for some years.  So people do seem to be
> > exercising appropriate restraint here.
> 
> Or people just don't know about it :-)

"Ignorance: The #1 contributor to appropriate restraint!"  ;-)

> > >    Because as soon as the option exists, people will use it for other
> > >    things too.
> > > 
> > >  - The proposed interface is very much exposed to everybody who wants
> > >    it; this again is wide open to (ab)use.
> > > 
> > >    Once it exists people will start to use, and before you know it we'll
> > >    always have that fast counter incremented and we're in IPI hell. Most
> > >    likely because someone was lazy and it seemed like a quick fix for
> > >    some stupid code.
> > 
> > I suppose that another way to keep it private would be to have the
> > declaration in both update.c and rcutorture.c.  This would mean that no
> > other file could invoke it, and should keep 0day happy.  It would mean
> > that the declarations would be duplicated, but worse things could happen.
> 
> Why do you need it for rcu torture? That can call the regular expedited
> call to exercise those rcu paths, right?

Yes, but not the ability to turn expediting on and off in the normal path.

> That would allow you to use system_state < SYSTEM_RUNNING if you really
> wanted to do this without exposing any interface for this.

This decision is not up to RCU.  Something else must tell RCU whether or
not and when to treat normal grace periods as expedited.

							Thanx, Paul

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