From: Sergey Senozhatsky <senozhatsky@chromium.org>
To: Petr Mladek <pmladek@suse.com>
Cc: Luo Jiaxing <luojiaxing@huawei.com>,
sergey.senozhatsky@gmail.com, rostedt@goodmis.org,
john.ogness@linutronix.de, linux-kernel@vger.kernel.org,
Sergey Senozhatsky <senozhatsky@chromium.org>,
linuxarm@huawei.com, bobo.shaobowang@huawei.com
Subject: Re: [PATCH] printk: stop spining waiter when console resume to flush prb
Date: Thu, 6 May 2021 23:07:19 +0900 [thread overview]
Message-ID: <YJP4F1UIt/eRZ96s@google.com> (raw)
In-Reply-To: <YJPxj83F1sBjHHAE@alley>
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.
next prev parent reply other threads:[~2021-05-06 14:07 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-06 8:00 [PATCH] printk: stop spining waiter when console resume to flush prb Luo Jiaxing
2021-05-06 13:13 ` Steven Rostedt
2021-05-07 8:35 ` luojiaxing
2021-05-06 13:39 ` Petr Mladek
2021-05-06 14:07 ` Sergey Senozhatsky [this message]
2021-05-06 14:12 ` Sergey Senozhatsky
2021-05-06 15:14 ` John Ogness
2021-05-07 7:58 ` luojiaxing
2021-05-07 7:33 ` luojiaxing
2021-05-07 7:49 ` Sergey Senozhatsky
2021-05-07 16:36 ` Petr Mladek
2021-05-10 8:26 ` Sergey Senozhatsky
2021-05-10 10:17 ` Petr Mladek
2021-05-10 10:32 ` John Ogness
2021-05-10 11:16 ` Sergey Senozhatsky
2021-05-10 11:43 ` Sergey Senozhatsky
2021-05-07 16:13 ` Petr Mladek
2021-05-10 8:29 ` luojiaxing
2021-05-10 9:50 ` Petr Mladek
2021-05-10 12:06 ` Sergey Senozhatsky
2021-05-10 7:41 ` luojiaxing
2021-05-10 9:30 ` Petr Mladek
2021-05-11 7:32 ` luojiaxing
2021-05-11 9:08 ` Petr Mladek
2021-05-13 7:55 ` luojiaxing
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