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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