linux-kernel - Re: POC: Alternative solution: Re: [PATCH 0/4] printk: reimplement LOG

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Message-ID: <20200813073158.GI12903@alley>
Date:   Thu, 13 Aug 2020 09:31:58 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        kexec@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: POC: Alternative solution: Re: [PATCH 0/4] printk: reimplement
 LOG_CONT handling

On Thu 2020-08-13 02:30:02, John Ogness wrote:
> On 2020-08-12, Petr Mladek <pmladek@...e.com> wrote:
> > So, I have one crazy idea to add one more state bit so that we
> > could have:
> >
> >   + committed: set when the data are written into the data ring.
> >   + final: set when the data block could not longer get reopened
> >   + reuse: set when the desctiptor/data block could get reused
> >
> > "final" bit will define when the descriptor could not longer
> > get reopened (cleared committed bit) and the data block could
> > not get extended.
> 
> I implemented this solution for myself and am currently running more
> tests. Some things that I changed from your suggestion:
> 
> 1. I created a separate prb_reserve_cont() function. The reason for this
> is because the caller needs to understand what is happening. The caller
> is getting an existing record with existing data and must append new
> data. The @text_len field of the info reports how long the existing data
> is.

I see.

> So the LOG_CONT handling code in printk.c looks something like this:

Yeah, it makes sense.


> 2. I haven't yet figured out how to preserve calling context when a
> newline appears. For example:
> 
> pr_info("text");
> pr_cont(" 1");
> pr_cont(" 2\n");
> pr_cont("3");
> pr_cont(" 4\n");
> 
> For "3" the calling context (info, timestamp) is lost because with "2"
> the record is finalized. Perhaps the above is invalid usage of LOG_CONT?

I am going to answer Sergey's reply.


> 3. There are some memory barriers introduced, but it looks like it
> shouldn't add too much complexity.

Uff. I have hooped for this.

> I will continue to refine my working version and post a patch so that we
> have something to work with. This looks to be the most promising way
> forward. Thanks.

Uff, I am happy that it seems working. Several other approaches looked
much more complicated or they caused regressions.

Best Regards,
Petr