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

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Date:	Sun, 18 Aug 2013 11:34:44 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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 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... :-)

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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