On 13.02.2014 19:25, Peter Zijlstra wrote:
> On Thu, Feb 13, 2014 at 06:00:19PM +0100, Stefan Bader wrote:
>> 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?
>
> So the assembly merges the first and second should_resched(), so its
> possible that load got before the interrupt().
>
> The 3rd preempt_count load gets re-issued and so that would show the
> 'true' value again.
>
> If you want to force a reload after the condition; put in a barrier().
>
> In any case; this looks like a false-positive. Please try again until
> you get one where the interrupt doesn't happen and we stay in 'n' state.
>
Oh and one thing I was wondering. Not sure I do understand it right... When
initially converting to percpu counts, you changed the 32bit assembly like that:
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -362,12 +362,9 @@ END(ret_from_exception)
#ifdef CONFIG_PREEMPT
ENTRY(resume_kernel)
DISABLE_INTERRUPTS(CLBR_ANY)
- cmpl $0,TI_preempt_count(%ebp) # non-zero preempt_count ?
- jnz restore_all
need_resched:
- movl TI_flags(%ebp), %ecx # need_resched set ?
- testb $_TIF_NEED_RESCHED, %cl
- jz restore_all
+ cmpl $0,PER_CPU_VAR(__preempt_count)
+ jnz restore_all
testl $X86_EFLAGS_IF,PT_EFLAGS(%esp) # interrupts off (exception path
jz restore_all
call preempt_schedule_irq
This seems to say if preempt_count was 0 then then if the thread flag was set
and interrupts were not off(?) it would do a preempt ipi and then come back to
re-check the thread flag.
This would now be if preempt_count is 0 only... and I wonder whether that would
change from doing that loop...
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