On Mon, Mar 28, 2011 at 06:24:36AM -0700, Paul E. McKenney wrote: > On Mon, Mar 28, 2011 at 02:33:36PM +0200, Sedat Dilek wrote: > > On Mon, Mar 28, 2011 at 6:08 AM, Paul E. McKenney > > <paulmck@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> > > >> >> > > > >> >> > 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. > > Very good! > > > >> 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 > > OK, I was extracting a tarball and then removing the resulting source > tree. I will try this. Though it does seem strange -- I can understand > how removing a file tree would stress RCU, but not creating a tarball. > Ah, well, if I fully understood it, there would not be a bug. > > > ...plus parallelly opening 20 tabs in firefox. > > That's normally enough to get my system freaky and see RCU related > > messages in the logs. > > Hmmm... My normal test systems don't have X -- I will need to set > this up. > > > > 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. > > OK. > > > >> [ 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. > > OK, thank you for the info. > > > > 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. ) > > If TREE_RCU hits problems faster, it is probably best to stay with > TREE_RCU. And of course, one exception to this advice is if TREE_RCU hangs so hard and fast that you don't have time to get any diagnostics. If this is the case, then TREE_PREEMPT_RCU might be more productive. Thanx, Paul > > 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 > > Yep. > > > 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? > > Not at the moment. I will be looking at diagnostics will going > through the code, so might have something later. > > > BTW, today's linux-next (next-20110328) is still freaky, I applied the > > revert-rcu-patches patchset and all is fine. > > I reverted back to the commit preceding the one you pointed out last night > my time, so the upcoming -next should be less freaky. > > > - Sedat - > > > > P.S.: Note to myself > > > > # mount -t debugfs none /sys/kernel/debug/ > > # ln -s /sys/kernel/debug /debug > > > > # find /debug -name rcu > > Or: > > # cd /debug/rcu > > then dump out everything except for the .csv file (which is the same > as the non-.csv equivalent, but in spreadsheet format -- intended > for systems with 100s or 1000s of CPUs). > > Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
