On Fri, 2015-03-27 at 19:07 +0200, Purcareata Bogdan wrote: > On 27.02.2015 03:05, Scott Wood wrote: > > On Thu, 2015-02-26 at 14:31 +0100, Sebastian Andrzej Siewior wrote: > >> On 02/26/2015 02:02 PM, Paolo Bonzini wrote: > >>> > >>> > >>> On 24/02/2015 00:27, Scott Wood wrote: > >>>> This isn't a host PIC driver. It's guest PIC emulation, some of which > >>>> is indeed not suitable for a rawlock (in particular, openpic_update_irq > >>>> which loops on the number of vcpus, with a loop body that calls > >>>> IRQ_check() which loops over all pending IRQs). > >>> > >>> The question is what behavior is wanted of code that isn't quite > >>> RT-ready. What is preferred, bugs or bad latency? > >>> > >>> If the answer is bad latency (which can be avoided simply by not running > >>> KVM on a RT kernel in production), patch 1 can be applied. If the > >> can be applied *but* makes no difference if applied or not. > >> > >>> answer is bugs, patch 1 is not upstream material. > >>> > >>> I myself prefer to have bad latency; if something takes a spinlock in > >>> atomic context, that spinlock should be raw. If it hurts (latency), > >>> don't do it (use the affected code). > >> > >> The problem, that is fixed by this s/spin_lock/raw_spin_lock/, exists > >> only in -RT. There is no change upstream. In general we fix such things > >> in -RT first and forward the patches upstream if possible. This convert > >> thingy would be possible. > >> Bug fixing comes before latency no matter if RT or not. Converting > >> every lock into a rawlock is not always the answer. > >> Last thing I read from Scott is that he is not entirely sure if this is > >> the right approach or not and patch #1 was not acked-by him either. > >> > >> So for now I wait for Scott's feedback and maybe a backtrace :) > > > > Obviously leaving it in a buggy state is not what we want -- but I lean > > towards a short term "fix" of putting "depends on !PREEMPT_RT" on the > > in-kernel MPIC emulation (which is itself just an optimization -- you > > can still use KVM without it). This way people don't enable it with RT > > without being aware of the issue, and there's more of an incentive to > > fix it properly. > > > > I'll let Bogdan supply the backtrace. > > So about the backtrace. Wasn't really sure how to "catch" this, so what > I did was to start a 24 VCPUs guest on a 24 CPU board, and in the guest > run 24 netperf flows with an external back to back board of the same > kind. I assumed this would provide the sufficient VCPUs and external > interrupt to expose an alleged culprit. > > With regards to measuring the latency, I thought of using ftrace, > specifically the preemptirqsoff latency histogram. Unfortunately, I > wasn't able to capture any major differences between running a guest > with in-kernel MPIC emulation (with the openpic raw_spinlock_conversion > applied) vs. no in-kernel MPIC emulation. Function profiling > (trace_stat) shows that in the second case there's a far greater time > spent in kvm_handle_exit (100x), but overall, the maximum latencies for > preemptirqsoff don't look that much different. > > Here are the max numbers (preemptirqsoff) for the 24 CPUs, on the host > RT Linux, sorted in descending order, expressed in microseconds: > > In-kernel MPIC QEMU MPIC > 3975 5105 What are you measuring? Latency in the host, or in the guest? -Scott -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
