linux-kernel - Re: [RFC PATCH] printk: Change timestamp to triplet as mono, boot and real

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Date:   Thu, 13 Aug 2020 10:55:00 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Orson Zhai <orsonzhai@...il.com>,
        Prarit Bhargava <prarit@...hat.com>,
        Dave Young <dyoung@...hat.com>, Baoquan He <bhe@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org, zhang.lyra@...il.com,
        ruifeng.zhang1@...soc.com, cixi.geng1@...soc.com,
        Orson Zhai <orson.zhai@...soc.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Steven Sistare <steven.sistare@...cle.com>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Jon DeVree <nuxi@...lt24.org>,
        Salvatore Bonaccorso <carnil@...ian.org>,
        John Ogness <john.ogness@...utronix.de>
Subject: Re: [RFC PATCH] printk: Change timestamp to triplet as mono, boot
 and real

On (20/08/11 15:02), Petr Mladek wrote:
> On Tue 2020-08-11 14:05:12, Thomas Gleixner wrote:
> > Petr Mladek <pmladek@...e.com> writes:
> > > At least "crash" tool would need an update anyway. AFAIK, it checks
> > > the size of struct printk_log and refuses to read it when it changes.
> > >
> > > It means that the hack with VMCOREINFO_FIELD_OFFSET probably is not
> > > needed because we would need to update the crashdump-related tools anyway.
> > >
> > > Well, the timing is good. We are about to switch the printk ring
> > > buffer into a lockless one. It requires updating the crashdump tools
> > > as well. We could do this at the same time. The lockless ring buffer
> > > already is in linux-next. It is aimed for 5.10 or 5.11.
> > ...
> > > It would be great to synchronize all these changes changes of the
> > > printk log buffer structures.
> > 
> > I agree that having one update is a good thing, but pretty please can we
> > finally make progress with this and not create yet another dependency?
> 
> To make it clear. I definitely do not want to block lockless printk by
> this.
> 
> BTW: I am not 100% convinced that storing all three timestamps is
> worth it. It increases the code complexity, metadata size. It needs
> an interface with the userspace that has to stay backward compatible.

Can we, perhaps, store those various "alternative" timestamps in dict so
then whoever wants to read them can just parse the dict key:value pairs
attach to each printk message?

> Also it still will be racy because the timestamp is taken when the message
> is printed. It might be "long" before or after the event that
> it talks about.
>
> There is still the alternative to print all three timestamps regularly
> for those interested. It is less user convenient but much easier
> to maintain.

Yes, that's a nice alternative.

	-ss