linux-kernel - Re: [RFC PATCH v4 0/9] printk: new ringbuffer implementation

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Date:   Fri, 6 Sep 2019 22:44:39 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        John Ogness <john.ogness@...utronix.de>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 0/9] printk: new ringbuffer implementation

On (09/06/19 12:49), Peter Zijlstra wrote:
> On Fri, Sep 06, 2019 at 07:09:43PM +0900, Sergey Senozhatsky wrote:
> 
> > ---
> > diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
> > index 139c310049b1..9c73eb6259ce 100644
> > --- a/kernel/printk/printk_safe.c
> > +++ b/kernel/printk/printk_safe.c
> > @@ -103,7 +103,10 @@ static __printf(2, 0) int printk_safe_log_store(struct printk_safe_seq_buf *s,
> >         if (atomic_cmpxchg(&s->len, len, len + add) != len)
> >                 goto again;
> >  
> > -       queue_flush_work(s);
> > +       if (early_console)
> > +               early_console->write(early_console, s->buffer + len, add);
> > +       else
> > +               queue_flush_work(s);
> >         return add;
> >  }
> 
> You've not been following along, that generates absolutely unreadable
> garbage.

This was more of a joke/reference to "Those NMI buffers are a trainwreck
and need to die a horrible death". Of course this needs a re-entrant cpu
lock to serialize access to atomic/early consoles. But here is one more
missing thing - we need atomic/early consoles on a separate, sort of
immutable, list. And probably forbid any modifications of such console
drivers, (PM, etc.) If we can do this then we don't need to take console_sem
while we iterate that list, which removes sched/timekeeping locks out
of the fast printk() path.

We, at the same time, don't have that many options on systems without
atomic/early consoles. Move printing to NMI (e.g. up to X pending logbug
lines per NMI)? Move printing to IPI (again, up to X pending logbuf lines
per IPI)? printk() softirqs?

	-ss