linux-kernel - Re: [RFC][PATCHv2 3/8] printk: offload printing from wake_up_klogd_work

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Message-ID: <20170331145602.GB3452@pathway.suse.cz>
Date:   Fri, 31 Mar 2017 16:56:02 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Eric Biederman <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>,
        Len Brown <len.brown@...el.com>, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv2 3/8] printk: offload printing from
 wake_up_klogd_work_func()

On Wed 2017-03-29 18:25:06, Sergey Senozhatsky wrote:
> Offload printing of printk_deferred() messages from IRQ context
> to a schedulable printing kthread, when possible (the same way
> we do it in vprintk_emit()). Otherwise, console_unlock() can
> force the printing CPU to spend unbound amount of time flushing
> kernel messages from IRQ context.
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> ---
>  kernel/printk/printk.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index ab6b3b2a68c6..1927b5cb5cbe 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2741,8 +2741,16 @@ static void wake_up_klogd_work_func(struct irq_work *irq_work)
>  		 * If trylock fails, someone else is doing the printing.
>  		 * PRINTK_PENDING_OUTPUT bit is cleared by console_unlock().
>  		 */
> -		if (console_trylock())
> -			console_unlock();
> +		if (printk_kthread_enabled()) {
> +			wake_up_process(printk_kthread);

Note that the relation between printk_kthread_enabled()
and wake_up_process() is racy. The conditions might change
between these two calls. It looks fine here, well almost.

The critical point is in vprintk_emit(). It must use the emergency
mode (call the consoles directly) when it is called from a process
that started the emergency mode.

We could be more relaxed here. IMHO, the only sensitive situation
is if printk_deferred() is used in the emergency context.
We might want to use the emergency mode here as well but
it is not guaranteed. printk_emergency might get cleared
in the meantime.

A solution might be to add one more bit, e.g.
PRINTK_PENDING_EMERGENCY_OUTPUT. We should force the emergency mode
here when it is set. It should be cleared together with the normal
PRINTK_PENDING_OUTPUT.

Or do you think that this is a corner case that we could
ignore for now?

Otherwise, it looks fine to me.

Best Regards,
Petr