On Thu, Apr 28, 2011 at 12:06:11AM +0200, Thomas Gleixner wrote: > On Wed, 27 Apr 2011, Bruno Prémont wrote: > > On Wed, 27 April 2011 Bruno Prémont wrote: > > Voluntary context switches stay constant from the time on SLABs pile up. > > (which makes sense as it doesn't run get CPU slices anymore) > > > > > > Can you please enable CONFIG_SCHED_DEBUG and provide the output of > > > > /proc/sched_stat when the problem surfaces and a minute after the > > > > first snapshot? > > > > hm, did you mean CONFIG_SCHEDSTAT or /proc/sched_debug? > > > > I did use CONFIG_SCHED_DEBUG (and there is no /proc/sched_stat) so I took > > /proc/sched_debug which exists... (attached, taken about 7min and +1min > > after SLABs started piling up), though build processes were SIGSTOPped > > during first minute. > > Oops. /proc/sched_debug is the right thing. > > > printk wrote (in case its timestamp is useful, more below): > > [ 518.480103] sched: RT throttling activated > > Ok. Aside of the fact that the CPU time accounting is completely hosed > this is pointing to the root cause of the problem. > > kthread_rcu seems to run in circles for whatever reason and the RT > throttler catches it. After that things go down the drain completely > as it should get on the CPU again after that 50ms throttling break. Ah. This could happen if there was a huge number of callbacks, in which case blimit would be set very large and kthread_rcu could then go CPU-bound. And this workload was generating large numbers of callbacks due to filesystem operations, right? So, perhaps I should kick kthread_rcu back to SCHED_NORMAL if blimit has been set high. Or have some throttling of my own. I must confess that throttling kthread_rcu for two hours seems a bit harsh. ;-) If this was just throttling kthread_rcu for a few hundred milliseconds, or even for a second or two, things would be just fine. Left to myself, I will put together a patch that puts callback processing down to SCHED_NORMAL in the case where there are huge numbers of callbacks to be processed. > Though we should not ignore the fact, that the RT throttler hit, but > none of the RT tasks actually accumulated runtime. > > So there is a couple of questions: > > - Why does the scheduler detect the 950 ms RT runtime, but does > not accumulate that runtime to any thread > > - Why is the runtime accounting totally hosed > > - Why does that not happen (at least not reproducible) with > TREE_RCU This one I can answer -- In Linus's tree, TREE_RCU still uses softirq, so there is no RCU kthread, so there is nothing to throttle other than ksoftirqd itself. Thanx, Paul > I need some sleep now, but I will try to come up with sensible > debugging tomorrow unless Paul or someone else beats me to it. > > Thanks, > > tglx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
