Re: [PATCH v3 2/2] Handle flushing of CPU backtraces during panic
From: takakura
Date: Thu Aug 15 2024 - 06:53:06 EST
Hi Petr and John,
On Tue 2024-08-13 13:45, Petr Mladek wrote:
>On Mon 2024-08-12 16:29:31, takakura@xxxxxxxxxxxxx wrote:
>> From: Ryo Takakura <takakura@xxxxxxxxxxxxx>
>>
>> After panic, non-panicked CPU's has been unable to flush ringbuffer
>> while they can still write into it. This can affect CPU backtrace
>> triggered in panic only able to write into ringbuffer incapable of
>> flushing them.
>
>I still think about this. The motivation is basically the same
>as in the commit 5d5e4522a7f404d1a96fd ("printk: restore flushing
>of NMI buffers on remote CPUs after NMI backtraces").
>
>It would make sense to replace printk_trigger_flush() with
>console_try_flush(). And the function should queue the irq
>work when it can't be flushed directly, see below.
Yes, I agree.
>> Fix the issue by letting the panicked CPU handle the flushing of
>> ringbuffer right after non-panicked CPUs finished writing their
>> backtraces.
>>
>> --- a/kernel/panic.c
>> +++ b/kernel/panic.c
>> @@ -260,6 +260,7 @@ static void panic_other_cpus_shutdown(bool crash_kexec)
>> panic_triggering_all_cpu_backtrace = true;
>> trigger_all_cpu_backtrace();
>> panic_triggering_all_cpu_backtrace = false;
>> + console_try_flush();
>> }
>>
>> /*
>> --- a/kernel/printk/printk.c
>> +++ b/kernel/printk/printk.c
>> @@ -3284,6 +3284,20 @@ void console_flush_on_panic(enum con_flush_mode mode)
>> console_flush_all(false, &next_seq, &handover);
>> }
>>
>> +/**
>> + * console_try_flush - try to flush consoles when safe
>> + *
>> + * Context: Any, except for NMI.
>
>It is safe even in NMI. is_printk_legacy_deferred() would return true
>in this case.
I'm sorry... I didn't notice this. Thanks.
>> + */
>> +void console_try_flush(void)
>> +{
>> + if (is_printk_legacy_deferred())
>> + return;
>> +
>> + if (console_trylock())
>> + console_unlock();
>> +}
>
>I would do something like:
>
>/**
> * console_try_or_trigger_flush - try to flush consoles directly when
> * safe or the trigger deferred flush.
> *
> * Context: Any
> */
>void console_try_or_trigger_flush(void)
>{
> if (!is_printk_legacy_deferred() && console_trylock())
> console_unlock();
> else
> defer_console_output();
>}
>
>and use it instead of printk_trigger_flush() in
>nmi_trigger_cpumask_backtrace().
Yes, that sounds better to me as well!
>Well, I would postpone this patch after we finalize the patchset
>adding con->write_atomic() callback. This patch depends on it anyway
>via is_printk_legacy_deferred(). The patchset might also add
>other wrappers for flushing consoles and we have to choose some
>reasonable names. Or John could integrate this patch into the patchset.
Sure, I can resend one once the patchset is finalized!
Or would it be better if I just leave it to John so that it can be part of
the patchset?
>Best Regards,
>Petr
Sincerely,
Ryo Takakura