Re: [PATCH v7 4/4] nmi_backtrace: generate one-line reports for idle cpus

Petr Mladek
2016-08-11T15:25:45Z

On Tue 2016-08-09 12:43:58, Chris Metcalf wrote:
> On 8/9/2016 8:43 AM, Petr Mladek wrote:
> >On Mon 2016-08-08 12:03:38, Chris Metcalf wrote:
> >>When doing an nmi backtrace of many cores, most of which are idle,
> >>the output is a little overwhelming and very uninformative. Suppress
> >>messages for cpus that are idling when they are interrupted and just
> >>emit one line, "NMI backtrace for N skipped: idling at pc 0xNNN".
> >Hmm, I do not see this message even though the CPU is in the idle state:
> >
> >[ 7918.884535] CPU: 3 PID: 0 Comm: swapper/3 Not tainted 4.8.0-rc1-4-default+ #3088
> >[ 7918.884538] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> >[ 7918.884539] task: ffff88013a594380 task.stack: ffff88013a598000
> >[ 7918.884541] RIP: 0010:[<ffffffff81050bc6>] [<ffffffff81050bc6>] native_safe_halt+0x6/0x10
> >[ 7918.884543] RSP: 0018:ffff88013a59bea8 EFLAGS: 00000206
> >[ 7918.884544] RAX: ffff88013a594380 RBX: 0000000000000003 RCX: 0000000000000000
> >[ 7918.884546] RDX: ffff88013a594380 RSI: 0000000000000001 RDI: ffff88013a594380
> >[ 7918.884548] RBP: ffff88013a59bea8 R08: 0000000000000000 R09: 0000000000000000
> >[ 7918.884550] R10: 0000000000000001 R11: 0000000000000001 R12: 0000000000000003
> >[ 7918.884551] R13: 0000000000000000 R14: ffff88013a598000 R15: ffff88013a598000
> >[ 7918.884553] FS: 0000000000000000(0000) GS:ffff88013fd80000(0000) knlGS:0000000000000000
> >[ 7918.884554] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >[ 7918.884556] CR2: 00007f8afc65e000 CR3: 00000001383b8000 CR4: 00000000000006e0
> >[ 7918.884557] Stack:
> >[ 7918.884559] ffff88013a59bec8 ffffffff819573d3 0000000000000003 0000000000000000
> >[ 7918.884561] ffff88013a59bed8 ffffffff8102628f ffff88013a59bee8 ffffffff819579ea
> >[ 7918.884562] ffff88013a59bf30 ffffffff810bfe1a ffff88013a598000 ffff88013a598000
> >[ 7918.884563] Call Trace:
> >[ 7918.884565] [<ffffffff819573d3>] default_idle+0x23/0x170
> >[ 7918.884566] [<ffffffff8102628f>] arch_cpu_idle+0xf/0x20
> >[ 7918.884568] [<ffffffff819579ea>] default_idle_call+0x2a/0x50
> >[ 7918.884570] [<ffffffff810bfe1a>] cpu_startup_entry+0x16a/0x260
> >[ 7918.884571] [<ffffffff8103faf6>] start_secondary+0xf6/0x100
> >[ 7918.884573] Code: 00 00 00 00 00 55 48 89 e5 fa 5d c3 66 0f 1f 84 00 00 00 00 00 55 48 89 e5 fb 5d c3 66 0f 1f 84 00 00 00 00 00 55 48 89 e5 fb f4 <5d> c3 0f 1f 84 00 00 00 00 00 55
> > 48 89 e5 f4 5d c3 66 0f 1f 84
> >
> >Note that I test it in a virtual machine using qemu.
> >
> >The strange thing is that I do not see .cpuidle.text section in
> >the vmlinux binary. But it is possible that I have misunderstood
> >the concept.
>
> The .cpuidle.text sections are merged into the final kernel's overall
> text segment. What you should see is something like this in the "nm -n"
> output from the built vmlinux:
>
> [...]
> ffffffff81922aa8 T __cpuidle_text_start
> ffffffff81922ab0 T default_idle
> ffffffff81922b90 t mwait_idle
> ffffffff81922d20 T acpi_processor_ffh_cstate_enter
> ffffffff81922df0 T default_idle_call
> ffffffff81922e30 t cpu_idle_poll
> ffffffff81922f50 t intel_idle
> ffffffff81923085 t acpi_idle_do_entry
> ffffffff819230d0 t poll_idle
> ffffffff81923143 T __cpuidle_text_end
> [...]
>
> In other words, all the cpuidle functions grouped together and bracketed by
> the __cpuidle_text_{start,end} symbols.
>
> Perhaps you were running with a kernel that didn't have the actual patch 4/4
> applied, but just the earlier patches? Or perhaps your host Linux has been
> patched, but not the guest Linux running in qemu? Or perhaps you are
> ending up doing an NMI backtrace on the host kernel, not the guest?

Hmm, the problem is that native_safe_halt() is called from default_idle()
here. The function is marked as inline but the compiler did not inline
it.

It helped me to put native_safe_halt() into the __cpuidle_text section:

diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
index b77f5edb03b0..e31d50acd491 100644
--- a/arch/x86/include/asm/irqflags.h
+++ b/arch/x86/include/asm/irqflags.h
@@ -44,7 +44,7 @@ static inline void native_irq_enable(void)
asm volatile("sti": : :"memory");
}

-static inline void native_safe_halt(void)
+static inline __attribute__((__section__(".cpuidle.text"))) void native_safe_halt(void)
{
asm volatile("sti; hlt": : :"memory");
}

I did not use __cpuidle macro because I was not able to include
linux/cpu.h into that header.

I wonder if it would be possible to detect the idle thread an other
way. For example, I wonder if it would be enough to check for the
PID 0.

Best Regards,
Petr