linux-kernel - Re: [RFC PATCH v1 00/25] printk: new implementation

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Date:   Wed, 13 Feb 2019 10:31:01 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 00/25] printk: new implementation

On (02/12/19 15:29), John Ogness wrote:
>
> 1. The printk buffer is protected by a global raw spinlock for readers
>    and writers. This restricts the contexts that are allowed to
>   access the buffer.

[..]

> 2. Because of #1, NMI and recursive contexts are handled by deferring
>    logging/printing to a spinlock-safe context. This means that
>    messages will not be visible if (for example) the kernel dies in
>    NMI context and the irq_work mechanism does not survive.

panic() calls printk_safe_flush_on_panic(), which iterates all per-CPU
buffers and moves data to the main logbuf; so then we can flush pending
logbuf message

	panic()
	  printk_safe_flush_on_panic();
	  console_flush_on_panic();

We don't really use irq_work mechanism for that.

> 3. Because of #1, when *not* using features such as PREEMPT_RT, large
>    latencies exist when printing to slow consoles.

Because of #1? I'm not familiar with PREEMPT_RT; but logbuf spinlock
should be unlocked while we print messages to slow consoles
(call_consoles_drivers() is protected by console_sem, not logbuf
lock).

So it's

	spin_lock_irqsave(logbuf);
	vsprintf();
	memcpy();
	spin_unlock_irqrestore(logbuf);

	console_trylock();
	for (;;)
	    call_console_drivers();
	    				// console_owner handover
	console_unlock();

Do you see large latencies because of logbuf spinlock?

> 5. Printing to consoles is the responsibility of the printk caller
>    and that caller may be required to print many messages that other
>    printk callers inserted. Because of this there can be enormous
>    variance in the runtime of a printk call.

That's complicated. Steven's console_owner handover patch makes
printk() more fair. We can have "winner takes it all" scenarios,
but significantly less often, IMO. Do you have any data that
suggest otherwise?

> 7. Loglevel INFO is handled the same as ERR. There seems to be an
>    endless effort to get printk to show _all_ messages as quickly as
>    possible in case of a panic (i.e. printing from any context), but
>    at the same time try not to have printk be too intrusive for the
>    callers. These are conflicting requirements that lead to a printk
>    implementation that does a sub-optimal job of satisfying both
>    sides.

Per my experience, fully preemptible "print it sometime maybe"
printk() does not work equally well for everyone.

	-ss