From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Jan Kara <jack@suse.com>, Tejun Heo <tj@kernel.org>,
Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
linux-kernel@vger.kernel.org,
Byungchul Park <byungchul.park@lge.com>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Jan Kara <jack@suse.cz>
Subject: Re: [RFC][PATCH v8 1/2] printk: Make printk() completely async
Date: Fri, 1 Apr 2016 10:08:03 +0900 [thread overview]
Message-ID: <20160401010803.GA501@swordfish> (raw)
In-Reply-To: <20160331111229.GB1023@pathway.suse.cz>
Hello Petr,
On (03/31/16 13:12), Petr Mladek wrote:
> > + * Set printing kthread sleep condition early, under the
> > + * logbuf_lock, so it (if RUNNING) will go to console_lock()
> > + * and spin on logbuf_lock.
> > + */
> > + if (!in_panic && printk_kthread && !need_flush_console)
> > + need_flush_console = true;
>
> I would remove the if-statement and always set it:
>
> + It does not matter if we set it in panic. It will not affect
> anything.
hm... yes, you're right.
> + The check for printk_kthread is racy. It might be false here
> and it might be true later when check whether to wakeup
> the kthread or try to get console directly.
which is fine, isn't it? we will wakeup the printing kthread, it will
console_lock()/console_unlock() (regardless the state of need_flush_console).
printing thread checks need_flush_console only when it decides whether
it shall schedule.
> + We might set it true even when it was true before.
>
> I think that this was an attempt to avoid a spurious wake up.
> But we solve it better way now.
we also may have 'printk.synchronous = 1', which will purposelessly
dirty need_flush_console from various CPUs from every printk /* and
upon every return from console_unlock() */; that's why I added both
printk_kthread and !need_flush_console (re-dirty already dirtied)
checks.
> > raw_spin_lock(&logbuf_lock);
> > retry = console_seq != log_next_seq;
> > + if (!retry && printk_kthread)
> > + need_flush_console = false;
>
> Similar here. I remove the if-statement and always set it. We will
> either retry or it should be false anyway.
well, 'printk.synchronous = 1'. seems that `!retry' check can be
dropped, I agree.
a side nano-note,
apart from 'printk.synchronous = 1', we also can have !printk_kthread
because kthread_run(printk_kthread_func) failed. it's quite unlikely,
but still.
[..]
> > + while (1) {
> > + set_current_state(TASK_INTERRUPTIBLE);
> > + if (!need_flush_console)
> > + schedule();
> > +
> > + __set_current_state(TASK_RUNNING);
>
>
> We still must do here:
>
> need_flush_console = false;
oh, wow! that's a major mistake. thanks a lot for catching this!
shame on me.
> /*
> * Avoid an infinite loop when console_unlock() cannot
> * access consoles, e.g. because of a suspend. We
> * could get asleep here. Someone else will call
> * consoles if conditions change.
> */
looks ok.
> Also another name might help. If we set it false here, the value
> does describe a global state. The variable describes if this
> kthread needs to flush the console. So, more precise name would be
>
> printk_kthread_need_flush_console
yes, makes sense.
> I think that we are close. I really like the current state of
> the patch and how minimalistic it is.
thanks for your review.
I'll re-spin.
-ss
next prev parent reply other threads:[~2016-04-01 1:06 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-24 15:43 [RFC][PATCH v8 0/2] printk: Make printk() completely async Sergey Senozhatsky
2016-03-24 15:43 ` [RFC][PATCH v8 1/2] " Sergey Senozhatsky
2016-03-29 9:14 ` Jan Kara
2016-04-01 1:14 ` Sergey Senozhatsky
2016-03-31 11:12 ` Petr Mladek
2016-03-31 11:52 ` Jan Kara
2016-04-04 9:41 ` Petr Mladek
2016-04-04 17:01 ` Sergey Senozhatsky
2016-04-01 1:08 ` Sergey Senozhatsky [this message]
2016-04-01 8:59 ` Petr Mladek
2016-04-01 9:36 ` Sergey Senozhatsky
2016-04-01 11:30 ` Petr Mladek
2016-03-24 15:43 ` [RFC][PATCH v8 2/2] printk: Make wake_up_klogd_work_func() async Sergey Senozhatsky
2016-03-31 11:14 ` Petr Mladek
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