linux-kernel - Re: 2.6.26-rc9-git4: Reported regressions from 2.6.25

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Message-ID: <20080801210958.GL14851@linux.vnet.ibm.com>
Date:	Fri, 1 Aug 2008 14:09:58 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Adrian Bunk <bunk@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Natalie Protasevich <protasnb@...il.com>,
	Kernel Testers List <kernel-testers@...r.kernel.org>
Subject: Re: 2.6.26-rc9-git4: Reported regressions from 2.6.25

On Fri, Jul 11, 2008 at 02:11:59PM +1000, Nick Piggin wrote:
> On Friday 11 July 2008 04:06, Ingo Molnar wrote:
> 
> > i'm wondering why rcutorture didnt trigger it. I do run !HOTPLUG +
> > RCU_PREEMPT kernels and never saw this. Nor did Paul. That aspect is
> > weird.
> 
> It basically requires an active rcu reader to be preempted (preferably
> by something doing a lot of call_rcu or other activity ie. the writer
> so it can tick along the different states quickly).
> 
> I found just 2 threads (reader and writer) bound to the same CPU would
> trigger it fastest, my reader has quite a long rcu read section.
> 
> I'm not sure why rcutorture doesn't trigger for everyone. I'm surprised
> it does not have much longer maximum read delays -- several ms I would
> have thought should be useful to have a crticial section open while the
> rcu engine can run through a number of states...

Hit it in 10 seconds once I actually got HOTPLUG_CPU disabled.

The theory behind the default settings for rcutorture are as follows:

o	Having two reader threads for each CPU helps ensure interactions
	between those threads.

o	The writer is normally going to have to share a CPU with a
	reader or two, maybe three.  This should force reader-writer
	interactions.

o	The read-hold time needs to be long enough to ensure interactions
	with the writer, but if it is too long, there are too few
	rcu_read_lock() and rcu_read_unlock() events to really stress
	the read-side processing.

o	The four fakewriters ensure interaction between multiple
	writers.

To Nick's point, I did use a hacked-up rcutorture with millisecond
read-side delays when debugging preemptable RCU, but I also used stock
rcutorture.

I will give this some thought and see if the defaults should change or
if more knobs are needed.

							Thanx, Paul
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