linux-kernel - Re: [tip:core/rcu] Revert "rcu: Decrease memory-barrier usage based on semi-formal proof"

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Message-ID: <20110526162802.GC2386@linux.vnet.ibm.com>
Date:	Thu, 26 May 2011 09:28:02 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	tglx@...utronix.de, mingo@...e.hu
Subject: Re: [tip:core/rcu] Revert "rcu: Decrease memory-barrier usage
 based on semi-formal proof"

On Thu, May 26, 2011 at 08:08:26AM -0700, Yinghai Lu wrote:
> On Wed, May 25, 2011 at 6:30 PM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> > On Wed, May 25, 2011 at 06:13:10PM -0700, Paul E. McKenney wrote:
> >> On Wed, May 25, 2011 at 03:49:25PM -0700, Yinghai Lu wrote:
> >> > On 05/25/2011 03:34 PM, Paul E. McKenney wrote:
> >> > > On Wed, May 25, 2011 at 03:15:50PM -0700, Yinghai Lu wrote:
> >> > >>> There is a new branch yinghai.2011.05.24a on:
> >> > >>>
> >> > >>> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git
> >> > >>>
> >> > >>> Or will be as soon as kernel.org updates its mirrors.
> >> > >>>
> >> > >>> I am not sure I could call this "clean", but it does revert that commit
> >> > >>> and 11 of the subsequent commits that depend on it.  It does build,
> >> > >>> and I will test it once my currently running tests complete.
> >> > >>
> >> > >> yes, with those revert, there is no delay in 10 times booting.
> >> > >
> >> > > Unfortunately, there are rcutorture test failures with the revert...
> >> >
> >> > confused.
> >>
> >> Given what I had to do to generate the revert, not exactly a surprise,
> >> I am afraid.  Just means that the resulting RCU sometimes fails to
> >> wait for all pre-existing readers, and rcutorture catches it.
> >>
> >> > what is the next?
> >>
> >> 1.    I send you a patch that I hope will fix the softlockup
> >>       you saw.  I am testing this.
> >>
> >> 2.    I am working on more detailed instrumentation, and will
> >>       send a patch on that.
> >>
> >> 3.    If time allows, break down the operations RCU is doing
> >>       and test them in isolation.
> >>
> >> Other thoughts?
> >
> > And here is patch #1.  Could you please try applying this on top of
> > Peter Zijlstra's patch to see if it gets rid of the softlockups you saw?
> >
> >                                                        Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > rcu: Start RCU kthreads in TASK_INTERRUPTIBLE state
> >
> > Upon creation, kthreads are in TASK_UNINTERRUPTIBLE state, which can
> > result in softlockup warnings.  Because some of RCU's kthreads can
> > legitimately be idle indefinitely, start them in TASK_INTERRUPTIBLE
> > state in order to avoid those warnings.
> 
> Yes, it fixes the lock up warning.

Very good, I have added your Tested-by.

							Thanx, Paul
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