RCU lockup issues when CONFIG_SOFTLOCKUP_DETECTOR=n - any one else seeing this?

[Jonathan Cameron]

On Mon, 31 Jul 2017 12:09:08 +0100
Jonathan Cameron <Jonathan.Cameron at huawei.com> wrote:

> On Wed, 26 Jul 2017 16:15:05 -0700
> "Paul E. McKenney" <paulmck at linux.vnet.ibm.com> wrote:
> 
> > On Wed, Jul 26, 2017 at 03:45:40PM -0700, David Miller wrote:  
> > > From: "Paul E. McKenney" <paulmck at linux.vnet.ibm.com>
> > > Date: Wed, 26 Jul 2017 15:36:58 -0700
> > >     
> > > > And without CONFIG_SOFTLOCKUP_DETECTOR, I see five runs of 24 with RCU
> > > > CPU stall warnings.  So it seems likely that CONFIG_SOFTLOCKUP_DETECTOR
> > > > really is having an effect.    
> > > 
> > > Thanks for all of the info Paul, I'll digest this and scan over the
> > > code myself.
> > > 
> > > Just out of curiousity, what x86 idle method is your machine using?
> > > The mwait one or the one which simply uses 'halt'?  The mwait variant
> > > might mask this bug, and halt would be a lot closer to how sparc64 and
> > > Jonathan's system operates.    
> > 
> > My kernel builds with CONFIG_INTEL_IDLE=n, which I believe means that
> > I am not using the mwait one.  Here is a grep for IDLE in my .config:
> > 
> > 	CONFIG_NO_HZ_IDLE=y
> > 	CONFIG_GENERIC_SMP_IDLE_THREAD=y
> > 	# CONFIG_IDLE_PAGE_TRACKING is not set
> > 	CONFIG_ACPI_PROCESSOR_IDLE=y
> > 	CONFIG_CPU_IDLE=y
> > 	# CONFIG_CPU_IDLE_GOV_LADDER is not set
> > 	CONFIG_CPU_IDLE_GOV_MENU=y
> > 	# CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED is not set
> > 	# CONFIG_INTEL_IDLE is not set
> >   
> > > On sparc64 the cpu yield we do in the idle loop sleeps the cpu.  It's
> > > local TICK register keeps advancing, and the local timer therefore
> > > will still trigger.  Also, any externally generated interrupts
> > > (including cross calls) will wake up the cpu as well.
> > > 
> > > The tick-sched code is really tricky wrt. NO_HZ even in the NO_HZ_IDLE
> > > case.  One of my running theories is that we miss scheduling a tick
> > > due to a race.  That would be consistent with the behavior we see
> > > in the RCU dumps, I think.    
> > 
> > But wouldn't you have to miss a -lot- of ticks to get an RCU CPU stall
> > warning?  By default, your grace period needs to extend for more than
> > 21 seconds (more than one-third of a -minute-) to get one.  Or do
> > you mean that the ticks get shut off now and forever, as opposed to
> > just losing one of them?
> >   
> > > Anyways, just a theory, and that's why I keep mentioning that commit
> > > about the revert of the revert (specifically
> > > 411fe24e6b7c283c3a1911450cdba6dd3aaea56e).
> > > 
> > > :-)    
> > 
> > I am running an overnight test in preparation for attempting to push
> > some fixes for regressions into 4.12, but will try reverting this
> > and enabling CONFIG_HZ_PERIODIC tomorrow.
> > 
> > Jonathan, might the commit that Dave points out above be what reduces
> > the probability of occurrence as you test older releases?  
> I just got around to trying this out of curiosity.  Superficially it did
> appear to possibly make the issue harder to hit took over 30 minutes
> but the issue otherwise looks much the same with or without that patch.
> 
> Just out of curiosity, next thing on my list is to disable hrtimers entirely
> and see what happens.
> 
> Jonathan
> > 
> > 							Thanx, Paul
> >   
> 
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