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). ÂNevertheless, 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. ÂThe >> > 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. ÂYes, >> > there might be other causes, but this is the most likely. ÂGiven that >> > TREE_PREEMPT_RCU behaves better than TREE_RCU, it is likely that this >> > loop is in preemptible code with irqs enabled. ÂTherefore, 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. ÂThis would narrow >> > the problem down to a few tens of lines of code. ÂIf 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. >> > >> > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â ÂThanx, Paul >> > >> >> - Sedat - >> >> >> >> >> >> >> >> > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â ÂThanx, 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. >> >> > >> >> > Â ÂSigned-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) >> >> > Â Â Â Âmod_timer(&yield_timer, jiffies + 2); >> >> > Â Â Â Âsp.sched_priority = 0; >> >> > Â Â Â Âsched_setscheduler_nocheck(current, SCHED_NORMAL, &sp); >> >> > + Â Â Â set_user_nice(current, 19); >> >> > Â Â Â Âschedule(); >> >> > Â Â Â Âsp.sched_priority = RCU_KTHREAD_PRIO; >> >> > Â Â Â Âsched_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. ÂAnd 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? ÂIs 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? ÂAre all of these running at normal > priority, or are some of them running at real-time priority? > Nothing special. >> [ setup.log ] >> ... >> Â (+) OK Â revert-rcu-patches/0001-Revert-rcu-introduce-kfree_rcu.patch >> Â (+) OK Â revert-rcu-patches/0002-Revert-rcu-fix-spelling.patch >> Â (+) OK Â revert-rcu-patches/0003-Revert-rcu-fix-rcu_cpu_kthread_task-synchronization.patch >> Â (+) OK Â revert-rcu-patches/0004-Revert-rcu-call-__rcu_read_unlock-in-exit_rcu-for-tr.patch >> Â (+) OK Â revert-rcu-patches/0005-Revert-rcu-Converge-TINY_RCU-expedited-and-normal-bo.patch >> Â (+) OK Â revert-rcu-patches/0006-Revert-rcu-remove-useless-boosted_this_gp-field.patch >> Â (+) OK Â revert-rcu-patches/0007-Revert-rcu-code-cleanups-in-TINY_RCU-priority-boosti.patch >> Â (+) OK Â revert-rcu-patches/0008-Revert-rcu-Switch-to-this_cpu-primitives.patch >> Â (+) OK Â revert-rcu-patches/0009-Revert-rcu-Use-WARN_ON_ONCE-for-DEBUG_OBJECTS_RCU_HE.patch >> Â (+) OK Â revert-rcu-patches/0010-Revert-rcu-Enable-DEBUG_OBJECTS_RCU_HEAD-from-PREEMP.patch >> Â (+) OK Â revert-rcu-patches/0011-Revert-rcu-Add-boosting-to-TREE_PREEMPT_RCU-tracing.patch >> Â (+) OK Â revert-rcu-patches/0012-Revert-rcu-eliminate-unused-boosting-statistics.patch >> Â (+) OK Â revert-rcu-patches/0013-Revert-rcu-priority-boosting-for-TREE_PREEMPT_RCU.patch >> Â (+) OK Â 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? ÂThe "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 -- 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
