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

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Message-ID: <20130514145119.GC19669@dyad.programming.kicks-ass.net>
Date:	Tue, 14 May 2013 16:51:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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

> 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?

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

>  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 ;-)
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