panic: Reduce CPU consumption after panic

[PATCH v2 2/2] x86/panic: Add x86_panic_handler as default post-panic behavior

Posted by carlos.bilbao@kernel.org 9 months, 2 weeks ago

From: Carlos Bilbao <carlos.bilbao@kernel.org>

Add function x86_panic_handler() as the default behavior for x86 for
post-panic stage via panic_set_handling(). Instead of busy-wait loop, it
will halt if there's no console to save CPU cycles.

Signed-off-by: Carlos Bilbao (DigitalOcean) <carlos.bilbao@kernel.org>
---
 arch/x86/kernel/setup.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 9d2a13b37833..3bfef55e9adb 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -16,6 +16,7 @@
 #include <linux/initrd.h>
 #include <linux/iscsi_ibft.h>
 #include <linux/memblock.h>
+#include <linux/panic.h>
 #include <linux/panic_notifier.h>
 #include <linux/pci.h>
 #include <linux/root_dev.h>
@@ -837,6 +838,15 @@ static void __init x86_report_nx(void)
 	}
 }
 
+
+static void x86_panic_handler(void)
+{
+	if (console_trylock()) {
+		console_unlock();
+		safe_halt();
+	}
+}
+
 /*
  * Determine if we were loaded by an EFI loader.  If so, then we have also been
  * passed the efi memmap, systab, etc., so we should use these data structures
@@ -1252,6 +1262,8 @@ void __init setup_arch(char **cmdline_p)
 #endif
 
 	unwind_init();
+
+	panic_set_handling(x86_panic_handler, 1);
 }
 
 #ifdef CONFIG_X86_32
-- 
2.47.1

Re: [PATCH v2 2/2] x86/panic: Add x86_panic_handler as default post-panic behavior

Posted by John Ogness 9 months, 2 weeks ago

On 2025-04-28, carlos.bilbao@kernel.org wrote:
> From: Carlos Bilbao <carlos.bilbao@kernel.org>
>
> Add function x86_panic_handler() as the default behavior for x86 for
> post-panic stage via panic_set_handling(). Instead of busy-wait loop, it
> will halt if there's no console to save CPU cycles.
>
> Signed-off-by: Carlos Bilbao (DigitalOcean) <carlos.bilbao@kernel.org>
> ---
>  arch/x86/kernel/setup.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 9d2a13b37833..3bfef55e9adb 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -16,6 +16,7 @@
>  #include <linux/initrd.h>
>  #include <linux/iscsi_ibft.h>
>  #include <linux/memblock.h>
> +#include <linux/panic.h>
>  #include <linux/panic_notifier.h>
>  #include <linux/pci.h>
>  #include <linux/root_dev.h>
> @@ -837,6 +838,15 @@ static void __init x86_report_nx(void)
>  	}
>  }
>  
> +
> +static void x86_panic_handler(void)
> +{
> +	if (console_trylock()) {
> +		console_unlock();
> +		safe_halt();
> +	}

I do not understand what you are trying to accomplish with the
console_trylock(). At this point in the panic, all the messages are
already output. The console lock is totally irrelevant.

Also, the console lock is only valid for legacy consoles.

I see no reason why you don't just use safe_halt() as your panic
handler.

John Ogness

Re: [PATCH v2 2/2] x86/panic: Add x86_panic_handler as default post-panic behavior

Posted by Carlos Bilbao 9 months, 2 weeks ago

Hello John,

On 4/29/25 11:53, John Ogness wrote:
> On 2025-04-28, carlos.bilbao@kernel.org wrote:
>> From: Carlos Bilbao <carlos.bilbao@kernel.org>
>>
>> Add function x86_panic_handler() as the default behavior for x86 for
>> post-panic stage via panic_set_handling(). Instead of busy-wait loop, it
>> will halt if there's no console to save CPU cycles.
>>
>> Signed-off-by: Carlos Bilbao (DigitalOcean) <carlos.bilbao@kernel.org>
>> ---
>>  arch/x86/kernel/setup.c | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>>
>> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
>> index 9d2a13b37833..3bfef55e9adb 100644
>> --- a/arch/x86/kernel/setup.c
>> +++ b/arch/x86/kernel/setup.c
>> @@ -16,6 +16,7 @@
>>  #include <linux/initrd.h>
>>  #include <linux/iscsi_ibft.h>
>>  #include <linux/memblock.h>
>> +#include <linux/panic.h>
>>  #include <linux/panic_notifier.h>
>>  #include <linux/pci.h>
>>  #include <linux/root_dev.h>
>> @@ -837,6 +838,15 @@ static void __init x86_report_nx(void)
>>  	}
>>  }
>>  
>> +
>> +static void x86_panic_handler(void)
>> +{
>> +	if (console_trylock()) {
>> +		console_unlock();
>> +		safe_halt();
>> +	}
> I do not understand what you are trying to accomplish with the
> console_trylock(). At this point in the panic, all the messages are
> already output. The console lock is totally irrelevant.
>
> Also, the console lock is only valid for legacy consoles.
>
> I see no reason why you don't just use safe_halt() as your panic
> handler.


Yes, in my original implementation I simply used halt, but I was trying to
be cautious in case any remaining messages hadn't flushed. I wonder, can we
be certain that, as you said, all output (e.g., a backtrace) has already
been displayed on screen at this point? I'm not sure.


>
> John Ogness


Thanks,

Carlos

Re: [PATCH v2 2/2] x86/panic: Add x86_panic_handler as default post-panic behavior

Posted by John Ogness 9 months, 2 weeks ago

On 2025-04-29, Carlos Bilbao <carlos.bilbao.osdev@gmail.com> wrote:
>> I see no reason why you don't just use safe_halt() as your panic
>> handler.
>
> Yes, in my original implementation I simply used halt, but I was
> trying to be cautious in case any remaining messages hadn't flushed. I
> wonder, can we be certain that, as you said, all output (e.g., a
> backtrace) has already been displayed on screen at this point? I'm not
> sure.

Well, without this series, the kernel goes into an infinite loop. So if
the messages are not all out, they won't magically appear during the
infinite loop either.

John

Re: [PATCH v2 2/2] x86/panic: Add x86_panic_handler as default post-panic behavior

Posted by Carlos Bilbao 9 months, 1 week ago

Hello,

On 4/30/25 02:58, John Ogness wrote:
> On 2025-04-29, Carlos Bilbao <carlos.bilbao.osdev@gmail.com> wrote:
>>> I see no reason why you don't just use safe_halt() as your panic
>>> handler.
>> Yes, in my original implementation I simply used halt, but I was
>> trying to be cautious in case any remaining messages hadn't flushed. I
>> wonder, can we be certain that, as you said, all output (e.g., a
>> backtrace) has already been displayed on screen at this point? I'm not
>> sure.
> Well, without this series, the kernel goes into an infinite loop. So if
> the messages are not all out, they won't magically appear during the
> infinite loop either.


I thought there may be some async stuff going on :)


>
> John


Thanks,

Carlos

Re: [PATCH v2 2/2] x86/panic: Add x86_panic_handler as default post-panic behavior

Posted by Sean Christopherson 9 months, 2 weeks ago

On Mon, Apr 28, 2025, carlos.bilbao@kernel.org wrote:
> From: Carlos Bilbao <carlos.bilbao@kernel.org>
> 
> Add function x86_panic_handler() as the default behavior for x86 for
> post-panic stage via panic_set_handling(). Instead of busy-wait loop, it
> will halt if there's no console to save CPU cycles.
> 
> Signed-off-by: Carlos Bilbao (DigitalOcean) <carlos.bilbao@kernel.org>
> ---
>  arch/x86/kernel/setup.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 9d2a13b37833..3bfef55e9adb 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -16,6 +16,7 @@
>  #include <linux/initrd.h>
>  #include <linux/iscsi_ibft.h>
>  #include <linux/memblock.h>
> +#include <linux/panic.h>
>  #include <linux/panic_notifier.h>
>  #include <linux/pci.h>
>  #include <linux/root_dev.h>
> @@ -837,6 +838,15 @@ static void __init x86_report_nx(void)
>  	}
>  }
>  
> +

Spurious newline.

> +static void x86_panic_handler(void)
> +{
> +	if (console_trylock()) {

A comment here would be very helpful.  Even with the changelog saying "if there's
no console", as an unfamiliar reader of console code, I have zero idea why being
able to lock the console is an effective test for no console.

> +		console_unlock();
> +		safe_halt();
> +	}
> +}

Re: [PATCH v2 2/2] x86/panic: Add x86_panic_handler as default post-panic behavior

Posted by Carlos Bilbao 9 months, 2 weeks ago

Hello,

On 4/29/25 09:57, Sean Christopherson wrote:
> On Mon, Apr 28, 2025, carlos.bilbao@kernel.org wrote:
>> From: Carlos Bilbao <carlos.bilbao@kernel.org>
>>
>> Add function x86_panic_handler() as the default behavior for x86 for
>> post-panic stage via panic_set_handling(). Instead of busy-wait loop, it
>> will halt if there's no console to save CPU cycles.
>>
>> Signed-off-by: Carlos Bilbao (DigitalOcean) <carlos.bilbao@kernel.org>
>> ---
>>  arch/x86/kernel/setup.c | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>>
>> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
>> index 9d2a13b37833..3bfef55e9adb 100644
>> --- a/arch/x86/kernel/setup.c
>> +++ b/arch/x86/kernel/setup.c
>> @@ -16,6 +16,7 @@
>>  #include <linux/initrd.h>
>>  #include <linux/iscsi_ibft.h>
>>  #include <linux/memblock.h>
>> +#include <linux/panic.h>
>>  #include <linux/panic_notifier.h>
>>  #include <linux/pci.h>
>>  #include <linux/root_dev.h>
>> @@ -837,6 +838,15 @@ static void __init x86_report_nx(void)
>>  	}
>>  }
>>  
>> +
> Spurious newline.
>
>> +static void x86_panic_handler(void)
>> +{
>> +	if (console_trylock()) {
> A comment here would be very helpful.  Even with the changelog saying "if there's
> no console", as an unfamiliar reader of console code, I have zero idea why being
> able to lock the console is an effective test for no console.


Agree, will fix in v3.


>
>> +		console_unlock();
>> +		safe_halt();
>> +	}
>> +}


Thanks,

Carlos