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

On Mon, Aug 21, 2017 at 10:52:58AM +1000, Nicholas Piggin wrote:

On Sun, 20 Aug 2017 14:14:29 -0700
"Paul E. McKenney" [off-list ref] wrote:

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On Sun, Aug 20, 2017 at 11:35:14AM -0700, Paul E. McKenney wrote:

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On Sun, Aug 20, 2017 at 11:00:40PM +1000, Nicholas Piggin wrote:  

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On Sun, 20 Aug 2017 14:45:53 +1000
Nicholas Piggin [off-list ref] wrote:
  

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On Wed, 16 Aug 2017 09:27:31 -0700
"Paul E. McKenney" [off-list ref] wrote:  

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On Wed, Aug 16, 2017 at 05:56:17AM -0700, Paul E. McKenney wrote:

Thomas, John, am I misinterpreting the timer trace event messages?    

So I did some digging, and what you find is that rcu_sched seems to do a
simple scheudle_timeout(1) and just goes out to lunch for many seconds.
The process_timeout timer never fires (when it finally does wake after
one of these events, it usually removes the timer with del_timer_sync).

So this patch seems to fix it. Testing, comments welcome.  

Okay this had a problem of trying to forward the timer from a timer
callback function.

This was my other approach which also fixes the RCU warnings, but it's
a little more complex. I reworked it a bit so the mod_timer fast path
hopefully doesn't have much more overhead (actually by reading jiffies
only when needed, it probably saves a load).  

Giving this one a whirl!  

No joy here, but then again there are other reasons to believe that I
am seeing a different bug than Dave and Jonathan are.

OK, not -entirely- without joy -- 10 of 14 runs were error-free, which
is a good improvement over 0 of 84 for your earlier patch.  ;-)  But
not statistically different from what I see without either patch.

But no statistical difference compared to without patch, and I still
see the "rcu_sched kthread starved" messages.  For whatever it is worth,
by the way, I also see this: "hrtimer: interrupt took 5712368 ns".
Hmmm...  I am also seeing that without any of your patches.  Might
be hypervisor preemption, I guess.

Okay it makes the warnings go away for me, but I'm just booting then
leaving the system idle. You're doing some CPU hotplug activity?

Yes, along with rcutorture, so a very different workload.

						Thanx, Paul