linux-kernel - Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing delay from HZ

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Date:	Tue, 14 May 2013 08:47:28 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu
Subject: Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing
 delay from HZ

On Tue, May 14, 2013 at 04:51:20PM +0200, Peter Zijlstra wrote:
> > In theory, yes.  In practice, this requires lots of lock acquisitions
> > and releases on large systems, including some global locks.  The weight
> > could be reduced, but...
> > 
> > What I would like to do instead would be to specify expedited grace
> > periods during boot.
> 
> But why, surely going idle without any RCU callbacks isn't completely unheard
> of, even outside of the boot process?

Yep, and RCU has special-cased that for quite some time.

> Being able to quickly drop out of the RCU state machinery would be a good thing IMO.

And this is currently possible -- this is the job of rcu_idle_enter()
and friends.  And it works well, at least when I get my "if" statements
set up correctly (hence the earlier patch).

Or are you seeing a slowdown even with that earlier patch applied?  If so,
please let me know what you are seeing.

> >  The challenge here appears to be somehow telling
> > RCU when boot is done.  The APIs are there from an RCU viewpoint:  boot
> > with rcupdate.rcu_expedited=1, then, once boot is complete (whatever
> > that means on your platform) "echo 0 > /sys/kernel/rcu_expedited".
> 
> Ha, and here you assume userspace is sane and co-operative. Fail in my book ;-)

If they are insane or uncooperative, they pay the price.  ;-)

							Thanx, Paul

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