Messages in this thread |  | | Date | Thu, 13 Feb 2014 19:03:56 +0100 | From | Stefan Bader <> | Subject | Re: Another preempt folding issue? |
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On 13.02.2014 18:38, 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? > > *blink*... That's weird indeed... do you have the asm that goes along > with that? > Yeah... not sure the interleaved source helps or not ...
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