linux-kernel - Re: [PATCH tip/core/rcu 01/11] rcu: Expedite grace periods during suspend/resume

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Message-ID: <20130819002910.GR29406@linux.vnet.ibm.com>
Date:	Sun, 18 Aug 2013 17:29:10 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Josh Triplett <josh@...htriplett.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH tip/core/rcu 01/11] rcu: Expedite grace periods during
 suspend/resume

On Sun, Aug 18, 2013 at 11:34:44AM +0200, Borislav Petkov wrote:
> On Sat, Aug 17, 2013 at 08:17:17PM -0700, Josh Triplett wrote:
> > On Sat, Aug 17, 2013 at 06:37:46PM -0700, Paul E. McKenney wrote:
> > > From: Borislav Petkov <bp@...en8.de>
> > > 
> > > CONFIG_RCU_FAST_NO_HZ can increase grace-period durations by up to
> > > a factor of four, which can result in long suspend and resume times.
> > > Thus, this commit temporarily switches to expedited grace periods when
> > > suspending the box and return to normal settings when resuming.
> > > 
> > > [ paulmck: This also papers over an audio/irq bug, but hopefully that will
> > >   be fixed soon. ]
> > > 
> > > Signed-off-by: Borislav Petkov <bp@...e.de>
> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > This patch still seems like a hack, and there *ought* to be a better
> > general solution to avoid excessive grace-period latency.  Nonetheless,
> > in the absence of such a solution,
> > Reviewed-by: Josh Triplett <josh@...htriplett.org>
> 
> Yeah, I'm not happy about it either but from quickly skimming over what
> context we're using rcu_expedited in, the basic requirement for a fix is
> for the pm core to be able to tell rcu not to stretch grace periods.
> 
> So simply setting a variable is much simpler than switching to calling
> all those *_expedited() rcu flavors from the pm notifier when going
> down.
> 
> Unless Paul has a better idea, of course.
> 
> The basic problem here is, however, that you need to temporarily
> reconfigure the inner workings of a subsystem because hardware is
> performing a power state transition. Unless someone teaches rcu about
> power state transitions... :-)

I would guess that once we have a few more subsystems that want RCU to
temporarily expedite its grace periods, we will have enough information
and experience to do something a bit more general.  In the meantime,
I am opting for something simple.  ;-)

							Thanx, Paul

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