linux-kernel - Re: [PATCH v2] printk: Relocate wake_klogd check close to the end of console

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Message-ID: <20180208145307.GA485@tigerII.localdomain>
Date:   Thu, 8 Feb 2018 23:53:07 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v2] printk: Relocate wake_klogd check close to the end of
 console_unlock()

On (02/08/18 14:04), Petr Mladek wrote:
> We mark for waking up klogd whenever we see a new message sequence in
> the main loop.  However, the actual wakeup is always at the end of the
> function and we can easily test for the wakeup condition when we do
> the final should-we-repeat check.
> 
> Move the wake_klogd condition check out of the main loop.  This avoids
> doing the same thing repeatedly and groups similar checks into a
> common place.
> 
> This fixes a race introduced by the commit dbdda842fe96f8932 ("printk: Add
> console owner and waiter logic to load balance console writes").
> The current console owner might process the newly added message before
> the related printk() start waiting for the console lock. Then the lock
> is passed without waking klogd. The new owner sees the already updated
> seen_seq and does not know that the wakeup is needed.

I need to do more "research" on this. I though about it some time ago,
and I think that waking up klogd _only_ when we don't have any pending
logbuf messages still can be pretty late. Can't it? We can spin in
console_unlock() printing loop for a long time, probably passing
console_sem ownership between CPUs, without waking up the log_wait waiter.
May be we can wake it up from the printing loop, outside of logbuf_lock,
and let klogd to compete for logbuf_lock with the printing CPU. Why do
we wake it up only when we are done pushing messages to a potentially
slow serial console?

	-ss