linux-kernel - Re: [RFC PATCH v4 9/9] printk: use a new ringbuffer implementation

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Message-ID: <87y2ztwiqh.fsf@linutronix.de>
Date:   Fri, 16 Aug 2019 11:40:54 +0200
From:   John Ogness <john.ogness@...utronix.de>
To:     Dave Young <dyoung@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        kexec@...ts.infradead.org, Simon Horman <horms@...ge.net.au>,
        Kazuhito Hagio <k-hagio@...jp.nec.com>
Subject: Re: [RFC PATCH v4 9/9] printk: use a new ringbuffer implementation

On 2019-08-16, Dave Young <dyoung@...hat.com> wrote:
> John, can you cc kexec list for your later series?

Sure.

> On 08/08/19 at 12:32am, John Ogness wrote:
>> This is a major change because the API (and underlying workings) of
>> the new ringbuffer are completely different than the previous
>> ringbuffer. Since there are several components of the printk
>> infrastructure that use the ringbuffer API (console, /dev/kmsg,
>> syslog, kmsg_dump), there are quite a few changes throughout the
>> printk implementation.
>> 
>> This is also a conservative change because it continues to use the
>> logbuf_lock raw spinlock even though the new ringbuffer is lockless.
>> 
>> The externally visible changes are:
>> 
>> 1. The exported vmcore info has changed:
>> 
>>     - VMCOREINFO_SYMBOL(log_buf);
>>     - VMCOREINFO_SYMBOL(log_buf_len);
>>     - VMCOREINFO_SYMBOL(log_first_idx);
>>     - VMCOREINFO_SYMBOL(clear_idx);
>>     - VMCOREINFO_SYMBOL(log_next_idx);
>>     + VMCOREINFO_SYMBOL(printk_rb_static);
>>     + VMCOREINFO_SYMBOL(printk_rb_dynamic);
>
> I assumed this needs some userspace work in kexec, how did you test
> them?

I did not test any direct userspace access to the ringbuffer structures.

> makedumpfile should need changes to dump the kernel log.
>
> Also kexec-tools includes a vmcore-dmesg.c to extrace dmesg from
> /proc/vmcore.

Thanks for the heads up. I'll take a look at it. The code changes should
be straight forward. I expect there will need to be backwards
compatibility. Perhaps it would check first for "printk_rb_*" then
fallback to "log_*"?

John Ogness