linux-kernel - Re: [PATCH] printk: stop spining waiter when console resume to flush prb

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Date:   Thu, 6 May 2021 23:07:19 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Luo Jiaxing <luojiaxing@...wei.com>, sergey.senozhatsky@...il.com,
        rostedt@...dmis.org, john.ogness@...utronix.de,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        linuxarm@...wei.com, bobo.shaobowang@...wei.com
Subject: Re: [PATCH] printk: stop spining waiter when console resume to flush
 prb

Thanks for Cc-ing Petr

On (21/05/06 15:39), Petr Mladek wrote:
> 
> Many printk messages might get accumulated when consoles were suspended.
> They are proceed when console_unlock() is called in resume_console().
> 
> The possibility to pass the console lock owner was added to reduce the risk
> of softlockup when too many messages were handled in an atomic context.
> 
> Now, resume_console() is always in a preemptible context that is safe
> to handle all accumulated messages. The possibility to pass the console
> lock owner actually makes things worse. The new owner might be in an atomic
> context and might cause softlockup when processing all messages accumulated
> when the console was suspended.
> 
> Create new console_unlock_preemptible() that will not allow to pass
> the console lock owner. As a result, all accumulated messages will
> be proceed in the safe preemptible process.

If we have a lot of pending messages in the logbuf, then there is
something chatty - some context (task, irq) or maybe several contexts.
And those contexts can continue adding messages, while we print them
_exclusively_ from preemptible context only. without ever throttling down
printk() callers - something that console_owner spinning and handover
does for us. And those printk() callers can even preempt
console_unlock_preemptible() and cause delays and lost messages.

In this regard, I'm afraid, console_unlock_preemptible() is somewhat
similar, IMHO, to the solution which we reverted - removal of
preempt_disable() before console_unlock() in vprintk_emit().

How about this.

Can we count the number of lines that we print from the `current` context
in console_unlock() and if after N messages there is no console_lock waiter
waiting for the `current` to handover console lock ownership, then create
one: schedule IRQ work that will become a console lock owner, spin on
console lock and call console_unlock() once it acquired the ownership.
That 'artificial' console lock owner will do the same - print N
messages, if nothing wants to become a console lock owner then it'll
queue another IRQ work.