linux-kernel - Re: [PATCH 2/4] printk: store instead of processing cont parts

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Date:   Tue, 21 Jul 2020 08:40:58 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...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: [PATCH 2/4] printk: store instead of processing cont parts

On Tue, Jul 21, 2020 at 7:42 AM Sergey Senozhatsky
<sergey.senozhatsky@...il.com> wrote:
>
> OK, so basically, extending printk_caller_id() so that for IRQ/NMI
> we will have more info than just "0x80000000 + raw_smp_processor_id()".

I think it's really preempt_count() that we want to have there.

That has the softirq/hardirq/NMI information in it.

So the "context" identifier of an incomplete write would be { task,
cpu, preempt_count() } of the writer. If a new KERN_CONT writer comes
in, it would have to match in that context to actually combine.

You can probably play "hide the bits" tricks to not *ac tually* use
three words for it. The task structure in particular tends to be very
aligned, you could hide bits in the low bits there. The CPU number
would fit in there, for example.

             Linus