linux-kernel - Re: [PATCH] printk: add cpu id information to printk() output

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Date:   Fri, 15 Sep 2023 11:53:13 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Enlin Mu <enlinmu@...il.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Enlin Mu <enlin.mu@...look.com>, pmladek@...e.com,
        rostedt@...dmis.org, senozhatsky@...omium.org,
        keescook@...omium.org, tony.luck@...el.com, gpiccoli@...lia.com,
        enlin.mu@...soc.com, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] printk: add cpu id information to printk() output

On Fri, Sep 15, 2023 at 04:46:02PM +0800, Enlin Mu wrote:
> John Ogness <john.ogness@...utronix.de> 于2023年9月15日周五 16:34写道：
> >
> > On 2023-09-15, Enlin Mu <enlin.mu@...look.com> wrote:
> > > Sometimes we want to print cpu id of printk() messages to consoles
> > >
> > > diff --git a/include/linux/threads.h b/include/linux/threads.h
> > > index c34173e6c5f1..6700bd9a174f 100644
> > > --- a/include/linux/threads.h
> > > +++ b/include/linux/threads.h
> > > @@ -34,6 +34,9 @@
> > >  #define PID_MAX_LIMIT (CONFIG_BASE_SMALL ? PAGE_SIZE * 8 : \
> > >       (sizeof(long) > 4 ? 4 * 1024 * 1024 : PID_MAX_DEFAULT))
> > >
> > > +#define CPU_ID_SHIFT 23
> > > +#define CPU_ID_MASK  0xff800000
> >
> > This only supports 256 CPUs. I think it doesn't make sense to try to
> > squish CPU and Task IDs into 32 bits.
> Yes, it is not good way,
> >
> > What about introducing a caller_id option to always only print the CPU
> > ID? Or do you really need Task _and_ CPU?
>    Yes, I need it.Because I need to know which CPU is printing the
> log, so that I can identify the current system operation, such as load
> situation and CPU busy/idle status

The cpu that is printing the log isn't the one that added the log
message, so I think you will have incorrect data here, right?

thanks,

greg k-h