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From:   Enlin Mu <enlinmu@gmail.com>
Date:   Fri, 22 Sep 2023 15:34:44 +0800
Message-ID: <CAAfh-jOnirbUjUYDhWqqUe8kPL=1FcDdZoSax5R-nGwO1CO60g@mail.gmail.com>
Subject: Re: [PATCH] printk: add cpu id information to printk() output
To:     John Ogness <john.ogness@linutronix.de>
Cc:     Enlin Mu <enlin.mu@outlook.com>, pmladek@suse.com,
        rostedt@goodmis.org, senozhatsky@chromium.org,
        keescook@chromium.org, tony.luck@intel.com, gpiccoli@igalia.com,
        enlin.mu@unisoc.com, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org
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John Ogness <john.ogness@linutronix.de> =E4=BA=8E2023=E5=B9=B49=E6=9C=8815=
=E6=97=A5=E5=91=A8=E4=BA=94 16:34=E5=86=99=E9=81=93=EF=BC=9A
>
> On 2023-09-15, Enlin Mu <enlin.mu@outlook.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.
>
> 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.
For SOC manufacturer, sometimes cpu is not stable, we need some debug
tools for this exceptions.
When an exception occurs, we may not be able to detect it in a timely
manner, but through Task _and_ CPU, we can roughly locate the CPU at
the time of the exception.

Thanks

Enlin

>
> John Ogness