Re: linux-next: Tree for March 25 (Call trace: RCU|workqueues|block|VFS|ext4 related?)

[Sedat Dilek <sedat.dilek@googlemail.com>]

On Mon, Mar 28, 2011 at 6:08 AM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Sun, Mar 27, 2011 at 11:48:30PM +0200, Sedat Dilek wrote:
>> On Sun, Mar 27, 2011 at 11:32 PM, Paul E. McKenney
>> <paulmck@linux.vnet.ibm.com> wrote:
>> > On Sun, Mar 27, 2011 at 02:26:15PM +0200, Sedat Dilek wrote:
>> >> On Sun, Mar 27, 2011 at 7:07 AM, Paul E. McKenney
>> >> <paulmck@linux.vnet.ibm.com> wrote:
>> >> > On Sat, Mar 26, 2011 at 08:25:29PM -0700, Paul E. McKenney wrote:
>> >> >> On Sun, Mar 27, 2011 at 03:30:34AM +0200, Sedat Dilek wrote:
>> >> >> > On Sun, Mar 27, 2011 at 1:09 AM, Paul E. McKenney
>> >> >> > <paulmck@linux.vnet.ibm.com> wrote:
>> >> >> > > On Sat, Mar 26, 2011 at 11:15:22PM +0100, Sedat Dilek wrote:
>> >> >
>> >> > [ . . . ]
>> >> >
>> >> >> > >> But then came RCU :-(.
>> >> >> > >
>> >> >> > > Well, if it turns out to be a problem in RCU I will certainly apologize.
>> >> >> > >
>> >> >> >
>> >> >> > No, that's not so dramatic.
>> >> >> > Dealing with this RCU issue has nice side-effects: I remembered (and
>> >> >> > finally did) to use a reduced kernel-config set.
>> >> >> > The base for it I created with 'make localmodconfig' and did some
>> >> >> > manual fine-tuning afterwards (throw out media, rc, dvd, unneeded FSs,
>> >> >> > etc.).
>> >> >> > Also, I can use fresh gcc-4.6 (4.6.0-1) from the official Debian repos.
>> >> >> >
>> >> >> > So, I started building with
>> >> >> > "revert-rcu-patches/0001-Revert-rcu-introduce-kfree_rcu.patch".
>> >> >> > I will let you know.
>> >> >>
>> >> >> And please also check for tasks consuming all available CPU.
>> >> >
>> >> > And I still cannot reproduce with the full RCU stack (but based off of
>> >> > 2.6.38 rather than -next).  Nevertheless, if you would like to try a
>> >> > speculative patch, here you go.
>> >>
>> >> You are right and my strategy on handling the (possible RCU?) issue is wrong.
>> >> Surely, you tested your RCU stuff in your own repo and everything
>> >> might be OK on top of stable 2.6.38.
>> >> Linux-next gets daily updates from a lot of different trees, so there
>> >> might be interferences with other stuff.
>> >> Please, understand I am interested in finding out what is the cause
>> >> for my issues, my aim is not to blame you.
>> >
>> > I am not worried about blame, but rather getting the bug fixed.  The
>> > bug might be in RCU, it might be elsewhere, or it might be a combination
>> > of problems in RCU and elsewhere.
>> >
>> > So the first priority is locating the bug.
>> >
>> > And that is why I have been asking you over and over to PLEASE take
>> > a look at what tasks are consuming CPU while the problem is occuring.
>> > The reason that I have been asking over and over is that the symptoms
>> > you describe are likely caused by a loop in some kernel code.  Yes,
>> > there might be other causes, but this is the most likely.  Given that
>> > TREE_PREEMPT_RCU behaves better than TREE_RCU, it is likely that this
>> > loop is in preemptible code with irqs enabled.  Therefore, the process
>> > accounting code is likely to be able to see the CPU consumption, and
>> > you should be able to see it via the "top" or "ps" commands -- or via
>> > any number of other tools.
>> >
>> > For example, if the problem is confined to RCU, you would likely see
>> > the "rcuc0" or "rcun0" tasks consuming lots of CPU.  This would narrow
>> > the problem down to a few tens of lines of code.  If the problem was
>> > in some other kthread, then identifying the kthread would very likely
>> > narrow things down as well.
>> >
>> > So, please do take a look to see what taks consuming CPU.
>> >
>> >> As I was wrong and want to be 99.9% sure it is RCU stuff, I reverted
>> >> all (18) RCU patches from linux-next (next-20110325) by keeping the
>> >> RCU|PREEMPT|HZ settings from last working next-20110323.
>> >
>> > Makes sense.
>> >
>> >> $ egrep 'RCU|PREEMPT|_HZ' /boot/config-2.6.38-next20110325-7-686-iniza
>> >> # RCU Subsystem
>> >> CONFIG_TREE_RCU=y
>> >> # CONFIG_PREEMPT_RCU is not set
>> >> # CONFIG_RCU_TRACE is not set
>> >> CONFIG_RCU_FANOUT=32
>> >> # CONFIG_RCU_FANOUT_EXACT is not set
>> >> CONFIG_RCU_FAST_NO_HZ=y
>> >> # CONFIG_TREE_RCU_TRACE is not set
>> >> CONFIG_PREEMPT_NOTIFIERS=y
>> >> CONFIG_NO_HZ=y
>> >> # CONFIG_PREEMPT_NONE is not set
>> >> CONFIG_PREEMPT_VOLUNTARY=y
>> >> # CONFIG_PREEMPT is not set
>> >> # CONFIG_HZ_100 is not set
>> >> CONFIG_HZ_250=y
>> >> # CONFIG_HZ_300 is not set
>> >> # CONFIG_HZ_1000 is not set
>> >> CONFIG_HZ=250
>> >> # CONFIG_SPARSE_RCU_POINTER is not set
>> >> # CONFIG_RCU_TORTURE_TEST is not set
>> >> # CONFIG_RCU_CPU_STALL_DETECTOR is not set
>> >>
>> >> I will work and stress this kernel before doing any step-by-step
>> >> revert of RCU stuff.
>> >>
>> >> Thanks for your patch, I applied it on top of "naked" next-20110325,
>> >> but I still see call-traces.
>> >
>> > Thank you very much for testing it!
>> >
>> > I intend to keep that patch, as it should increase robustness in other
>> > situations.
>> >
>> >                                                        Thanx, Paul
>> >
>> >> - Sedat -
>> >>
>> >>
>> >>
>> >> >                                                        Thanx, Paul
>> >> >
>> >> > ------------------------------------------------------------------------
>> >> >
>> >> > rcu: further lower priority in rcu_yield()
>> >> >
>> >> > Although rcu_yield() dropped from real-time to normal priority, there
>> >> > is always the possibility that the competing tasks have been niced.
>> >> > So nice to 19 in rcu_yield() to help ensure that other tasks have a
>> >> > better chance of running.
>> >> >
>> >> >    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>> >> >
>> >> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
>> >> > index 759f54b..5477764 100644
>> >> > --- a/kernel/rcutree.c
>> >> > +++ b/kernel/rcutree.c
>> >> > @@ -1492,6 +1492,7 @@ static void rcu_yield(void (*f)(unsigned long), unsigned long arg)
>> >> >        mod_timer(&yield_timer, jiffies + 2);
>> >> >        sp.sched_priority = 0;
>> >> >        sched_setscheduler_nocheck(current, SCHED_NORMAL, &sp);
>> >> > +       set_user_nice(current, 19);
>> >> >        schedule();
>> >> >        sp.sched_priority = RCU_KTHREAD_PRIO;
>> >> >        sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
>>
>> Sorry, my attempt was to identify and isolate the culprit commit.
>>
>> Reverting all RCU patches resulted in a stable system, the following 8
>> kernels with reduced k-config setup where all built using this kernel.
>>
>> All kernels used TREE_RCU (see above), I did not change it (no
>> mixing/switching to PREEMPT and TREE_PREEMPT_RCU).
>> ( I doubt that TREE_PREEMPT_RCU was some kind of more stable here. )
>>
>> The culprit commit is bc56163ebd4580199ac7e63f5e160bf139ba0dd6 (from
>> rcu/next GIT tree):
>> "rcu: move TREE_RCU from softirq to kthread"
>

Hi Paul,

> OK, please accept my apologies for your lost weekend.  And thank you for
> testing this.
>

No worries, it was mostly a rainy day.
The only thing I did @ 16:30 was to go to regional election (the new
(regional) prime minister will be the 1st from The German Green
party).

But back to RCU :-):
The reduced kernel-config setup decreased the build-time from approx.
2hrs (full, generic build) down to approx. 35mins.

>> I can do parallelly a tar job, open 20 tabs in firefox and run a flash
>> video in one of them (I did this several times).
>
> How many files in the tar job?  Is this creating a tar archive, expanding
> it, or both?
>

I am doing a simple tar (filesize: 1.6G for full and 1.0G for reduced build):

$ tar -cf $archivedir-on-external-usbhdd/$tarfile $kernel-build-dir

...plus parallelly opening 20 tabs in firefox.
That's normally enough to get my system freaky and see RCU related
messages in the logs.

> Do you have a script for this?  Are all of these running at normal
> priority, or are some of them running at real-time priority?
>

Nothing special.

>> [ setup.log ]
>> ...
>>   (+) OK   revert-rcu-patches/0001-Revert-rcu-introduce-kfree_rcu.patch
>>   (+) OK   revert-rcu-patches/0002-Revert-rcu-fix-spelling.patch
>>   (+) OK   revert-rcu-patches/0003-Revert-rcu-fix-rcu_cpu_kthread_task-synchronization.patch
>>   (+) OK   revert-rcu-patches/0004-Revert-rcu-call-__rcu_read_unlock-in-exit_rcu-for-tr.patch
>>   (+) OK   revert-rcu-patches/0005-Revert-rcu-Converge-TINY_RCU-expedited-and-normal-bo.patch
>>   (+) OK   revert-rcu-patches/0006-Revert-rcu-remove-useless-boosted_this_gp-field.patch
>>   (+) OK   revert-rcu-patches/0007-Revert-rcu-code-cleanups-in-TINY_RCU-priority-boosti.patch
>>   (+) OK   revert-rcu-patches/0008-Revert-rcu-Switch-to-this_cpu-primitives.patch
>>   (+) OK   revert-rcu-patches/0009-Revert-rcu-Use-WARN_ON_ONCE-for-DEBUG_OBJECTS_RCU_HE.patch
>>   (+) OK   revert-rcu-patches/0010-Revert-rcu-Enable-DEBUG_OBJECTS_RCU_HEAD-from-PREEMP.patch
>>   (+) OK   revert-rcu-patches/0011-Revert-rcu-Add-boosting-to-TREE_PREEMPT_RCU-tracing.patch
>>   (+) OK   revert-rcu-patches/0012-Revert-rcu-eliminate-unused-boosting-statistics.patch
>>   (+) OK   revert-rcu-patches/0013-Revert-rcu-priority-boosting-for-TREE_PREEMPT_RCU.patch
>>   (+) OK   revert-rcu-patches/0014-Revert-rcu-move-TREE_RCU-from-softirq-to-kthread.patch
>> ...
>>
>> Hope this helps to narrow down the problem.
>>
>> As I kept all kernels I can have a look at the tasks consuming high
>> CPU usage tomorrow.
>
> Could you please?
>

I recalled (as you say I requested over and over again from you :-)) I
looked with top, htop and 'ps axu', but there was nothing special.
Sometimes the system got frozen - at this point (or short before) I
did not see anything suspicious with top.

> Also, could you please mount debugfs and list out the files in the
> "rcu" directory?  The "ql=" value from the "rcu/rcudata" file is of
> particular interest.
>

Ah, before I forget...

I used TREE_RCU (was the default before noticing RCU issue) for
finding the culprit commit.
If it is from your POV more helpful to switch to PREEMPT + PREEMPT_RCU
+ RCU_BOOST, please let me *now* know.
( Both RCU setups freaks up the system. )

I think top & Co. are not enough to track the problem down.
I have seen tracing and debugging facililities for RCU.

Some questions to debug and trace setup:

Case #1: TREE_RCU

CONFIG_RCU_TRACE=y
CONFIG_TREE_RCU_TRACE=y

Case #2: PREEMPT + PREEMPT_RCU + RCU_BOOST

CONFIG_RCU_TRACE=y
CONFIG_TREE_RCU_TRACE=y
CONFIG_DEBUG_PREEMPT=y <--- Helpful?
CONFIG_PREEMPT_TRACER=y <--- Helpful?

Any other recommends for useful/helpful trace and/or debug options?

Any other intructions for debugging/tracing?

BTW, today's linux-next (next-20110328) is still freaky, I applied the
revert-rcu-patches patchset and all is fine.

- Sedat -

P.S.: Note to myself

# mount -t debugfs none /sys/kernel/debug/
# ln -s /sys/kernel/debug /debug

# find /debug -name rcu