Re: Another preempt folding issue?

[Stefan Bader <stefan.bader@canonical.com>]

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On 12.02.2014 12:54, Peter Zijlstra wrote:
> On Wed, Feb 12, 2014 at 12:09:29PM +0100, Stefan Bader wrote:
>> Something else here I run a kernel with CONFIG_PREEMPT not set and NR_CPUS
>> limited to 8 (for the 32bit kernel). So the default apic driver is used. Since
>> default_send_IPI_mask_logical is only used from there, I assume the trace you
>> got does the same. Maybe something there is wrong which would explain why we
>> only see it on 32bit hosts.
> 
> Can you try with a different APIC driver to test this?
> 
I don't think I can. And I think the statement about this only be used for 32bit
could be wrong. I got mislead to think so because those are only defined in
probe_32 but the 64bit counterpart isn't containing much aside that.

Anyway, I played around with tracing a bit more. So with this change:

                if (need_resched()) {
                        srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
                        if (need_resched() != should_resched()) {
+                               trace_printk("need(%i) != should(%i)\n",
+                                       need_resched(), should_resched());
+                               trace_printk("exit_reason=%u\n",
+                                       vcpu->run->exit_reason);
+                                trace_printk("preempt_count=%lx\n",
+                                        __this_cpu_read_4(__preempt_count));
+                                tracing_stop();
+                                printk(KERN_ERR "Stopped tracing, due to
inconsistent state.\n");
                        }
 +                      schedule();
 -                      cond_reschedule();
                        vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
                }

I get the following (weird) output:

            Xorg-1078  [001] d...    71.270251: native_smp_send_reschedule
<-resched_task
            Xorg-1078  [001] d...    71.270251: default_send_IPI_mask_logical
<-native_smp_send_reschedule
      bamfdaemon-2318  [001] d...    71.270465: resched_task <-check_preempt_wakeup
      bamfdaemon-2318  [001] d...    71.270539: resched_task <-check_preempt_wakeup
          compiz-2365  [001] d...    71.270689: resched_task <-check_preempt_wakeup
          compiz-2365  [001] d...    71.270827: resched_task <-check_preempt_wakeup
          compiz-2365  [001] d...    71.270940: resched_task <-check_preempt_wakeup
 qemu-system-x86-2679  [000] dn..    71.270999: smp_reschedule_interrupt
<-reschedule_interrupt
 qemu-system-x86-2679  [000] dn..    71.270999: scheduler_ipi
<-smp_reschedule_interrupt
 qemu-system-x86-2679  [000] .N..    71.271001: kvm_arch_vcpu_ioctl_run: need(1)
!= should(0)
 qemu-system-x86-2679  [000] .N..    71.271002: kvm_arch_vcpu_ioctl_run:
exit_reason=2
 qemu-system-x86-2679  [000] .N..    71.271003: kvm_arch_vcpu_ioctl_run:
preempt_count=0

So am I reading this right, that the interrupt did get delivered to cpu#0 while
the thread info already had the resched flag set. So this really should have
cleared the bit in preempt_count. But while the trace info shows 'N' for some
reason should_reschedule returns false but at the same time reading the preempt
count manually shows it 0?



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