From: Petr Mladek <pmladek@suse.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Tejun Heo <tj@kernel.org>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
akpm@linux-foundation.org, linux-mm@kvack.org,
Cong Wang <xiyou.wangcong@gmail.com>,
Dave Hansen <dave.hansen@intel.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>,
Vlastimil Babka <vbabka@suse.cz>,
Peter Zijlstra <peterz@infradead.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Jan Kara <jack@suse.cz>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
rostedt@home.goodmis.org, Byungchul Park <byungchul.park@lge.com>,
Pavel Machek <pavel@ucw.cz>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup
Date: Fri, 12 Jan 2018 13:55:37 +0100 [thread overview]
Message-ID: <20180112125536.GC24497@linux.suse> (raw)
In-Reply-To: <20180112072123.33bb567d@gandalf.local.home>
On Fri 2018-01-12 07:21:23, Steven Rostedt wrote:
> On Fri, 12 Jan 2018 19:05:44 +0900
> Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:
> > 3) console_unlock(void)
> > {
> > for (;;) {
> > printk_safe_enter_irqsave(flags);
> > // lock-unlock logbuf
> > call_console_drivers(ext_text, ext_len, text, len);
> > printk_safe_exit_irqrestore(flags);
> > }
> > }
> >
> > with slow serial console, call_console_drivers() takes enough time to
> > to make preemption of a current console_sem owner right after it irqrestore()
> > highly possible; unless there is a spinning console_waiter. which easily may
> > not be there; but can come in while current console_sem is preempted, why not.
> > so when preempted console_sem owner comes back - it suddenly has a whole bunch
> > of new messages to print and on one to hand off printing to. in a super
> > imperfect and ugly world, BTW, this is how console_unlock() still can be
> > O(infinite): schedule between the printed lines [even !PREEMPT kernel tries
>
> I'm not fixing console_unlock(), I'm fixing printk(). BTW, all my
> kernels are CONFIG_PREEMPT (I'm a RT guy), my mind thinks more about
> PREEMPT kernels than !PREEMPT ones.
I would say that the patch improves also console_unlock() but only in
non-preemttive context.
By other words, it makes console_unlock() finite in preemptible context
(limited by buffer size). It might still be unlimited in
non-preemtible context.
> > to cond_resched() after every line it prints] from current console_sem
> > owner and printk() while console_sem owner is scheduled out.
> >
> > 4) the interesting thing here is that call_console_drivers() can
> > cause console_sem owner to schedule even if it has handed off the
> > ownership. because waiting CPU has to spin with local IRQs disabled
> > as long as call_console_drivers() prints its message. so if consoles
> > are slow, then the first thing the waiter will face after it receives
> > the console_sem ownership and enables the IRQs is - preemption.
> > so hand off is not immediate. there is a possibility of re-scheduling
> > between hand off and actual printing. so that "there is always an active
> > printing CPU" is not quite true.
> >
> > vprintk_emit()
> > {
> >
> > console_trylock_spinning(void)
> > {
> > printk_safe_enter_irqsave(flags);
> > while (READ_ONCE(console_waiter)) // spins as long as call_console_drivers() on other CPU
> > cpu_relax();
> > printk_safe_exit_irqrestore(flags);
> > ---> }
> > | // preemptible up until printk_safe_enter_irqsave() in console_unlock()
> > | console_unlock()
> > | {
> > |
> > | ....
> > | for (;;) {
> > |--------------> printk_safe_enter_irqsave(flags);
> > ....
> > }
> >
> > }
> > }
> >
> > reverting 6b97a20d3a7909daa06625d4440c2c52d7bf08d7 may be the right
> > thing after all.
>
> I would analyze that more before doing so. Because with my patch, I
> think we make those that can do long prints (without triggering a
> watchdog), the ones most likely doing the long prints.
IMHO, it might make sense because it would help to see the messages
faster. But I would prefer to handle this separately because it
might also increase the risk of softlockups. Therefore it might
cause regressions.
We should also take into account the commit 8d91f8b15361dfb438ab6
("printk: do cond_resched() between lines while outputting to
consoles"). It has the same effect for console_lock() callers.
> > BTW, note the disclaimer [in capitals] -
> >
> > LIKE I SAID, IF STEVEN OR PETR WANT TO PUSH THE PATCH, I'M NOT
> > GOING TO BLOCK IT.
>
> GREAT! Then we can continue this conversation after the patch goes in.
> Because I'm focused on fixing #1 above.
Thanks for the disclaimer!
> > anyway. like I said weeks ago and repeated it in several emails: I have
> > no intention to NACK or block the patch.
> > but the patch is not doing enough. that's all I'm saying.
>
> Great, then Petr can start pushing this through.
>
> Below is my latest module I used for testing:
I am going to send v6 with fixes suggested for the 2nd patch by Steven.
Best Regards,
Petr
next prev parent reply other threads:[~2018-01-12 12:55 UTC|newest]
Thread overview: 140+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-10 13:24 [PATCH v5 0/2] printk: Console owner and waiter logic cleanup Petr Mladek
2018-01-10 13:24 ` [PATCH v5 1/2] printk: Add console owner and waiter logic to load balance console writes Petr Mladek
2018-01-10 16:50 ` Steven Rostedt
2018-01-12 16:54 ` Steven Rostedt
2018-01-12 17:11 ` Steven Rostedt
2018-01-17 19:13 ` Rasmus Villemoes
2018-01-17 19:33 ` Steven Rostedt
2018-01-19 9:51 ` Sergey Senozhatsky
2018-01-18 22:03 ` Pavel Machek
2018-01-19 0:20 ` Steven Rostedt
2018-01-17 2:19 ` Byungchul Park
2018-01-17 4:54 ` Byungchul Park
2018-01-17 7:34 ` Byungchul Park
2018-01-17 12:04 ` Petr Mladek
2018-01-18 1:53 ` Byungchul Park
2018-01-18 1:57 ` Byungchul Park
2018-01-18 2:19 ` Steven Rostedt
2018-01-18 4:01 ` Byungchul Park
2018-01-18 15:21 ` Steven Rostedt
2018-01-19 2:37 ` Byungchul Park
2018-01-19 3:27 ` Steven Rostedt
2018-01-22 2:31 ` Byungchul Park
2018-01-10 13:24 ` [PATCH v5 2/2] printk: Hide console waiter logic into helpers Petr Mladek
2018-01-10 17:52 ` Steven Rostedt
2018-01-11 12:03 ` Petr Mladek
2018-01-12 15:37 ` Steven Rostedt
2018-01-12 16:08 ` Petr Mladek
2018-01-12 16:36 ` Steven Rostedt
2018-01-15 16:08 ` Petr Mladek
2018-01-16 5:05 ` Sergey Senozhatsky
2018-01-10 14:05 ` [PATCH v5 0/2] printk: Console owner and waiter logic cleanup Tejun Heo
2018-01-10 16:29 ` Petr Mladek
2018-01-10 17:02 ` Tejun Heo
2018-01-10 18:21 ` Peter Zijlstra
2018-01-10 18:30 ` Tejun Heo
2018-01-10 18:41 ` Peter Zijlstra
2018-01-10 19:05 ` Tejun Heo
2018-01-11 5:15 ` Sergey Senozhatsky
2018-01-10 18:22 ` Steven Rostedt
2018-01-10 18:36 ` Tejun Heo
2018-01-10 18:40 ` Mathieu Desnoyers
2018-01-11 7:36 ` Sergey Senozhatsky
2018-01-11 11:24 ` Petr Mladek
2018-01-11 13:19 ` Sergey Senozhatsky
2018-01-24 9:36 ` Peter Zijlstra
2018-01-24 18:46 ` Tejun Heo
2018-05-09 8:58 ` Sergey Senozhatsky
2018-01-10 18:54 ` Steven Rostedt
2018-01-11 5:10 ` Sergey Senozhatsky
2018-01-10 18:05 ` Steven Rostedt
2018-01-10 18:12 ` Tejun Heo
2018-01-10 18:14 ` Tejun Heo
2018-01-10 18:45 ` Steven Rostedt
2018-01-10 18:41 ` Steven Rostedt
2018-01-10 18:57 ` Tejun Heo
2018-01-10 19:17 ` Steven Rostedt
2018-01-10 19:34 ` Tejun Heo
2018-01-10 19:44 ` Steven Rostedt
2018-01-10 22:44 ` Tejun Heo
2018-01-11 5:35 ` Sergey Senozhatsky
2018-01-11 4:58 ` Sergey Senozhatsky
2018-01-11 9:34 ` Petr Mladek
2018-01-11 10:38 ` Sergey Senozhatsky
2018-01-11 11:50 ` Petr Mladek
2018-01-11 16:29 ` Steven Rostedt
2018-01-12 1:30 ` Steven Rostedt
2018-01-12 2:55 ` Steven Rostedt
2018-01-12 4:20 ` Steven Rostedt
2018-01-16 19:44 ` Tejun Heo
2018-01-17 9:12 ` Petr Mladek
2018-01-17 15:15 ` Tejun Heo
2018-01-17 17:12 ` Steven Rostedt
2018-01-17 18:42 ` Steven Rostedt
2018-01-19 18:20 ` Steven Rostedt
2018-01-20 7:14 ` Sergey Senozhatsky
2018-01-20 15:49 ` Steven Rostedt
2018-01-21 14:15 ` Sergey Senozhatsky
2018-01-21 21:04 ` Steven Rostedt
2018-01-22 8:56 ` Sergey Senozhatsky
2018-01-22 10:28 ` Sergey Senozhatsky
2018-01-22 10:36 ` Sergey Senozhatsky
2018-01-23 6:40 ` Sergey Senozhatsky
2018-01-23 7:05 ` Sergey Senozhatsky
2018-01-23 7:31 ` Sergey Senozhatsky
2018-01-23 14:56 ` Steven Rostedt
2018-01-23 15:21 ` Sergey Senozhatsky
2018-01-23 15:41 ` Steven Rostedt
2018-01-23 15:43 ` Tejun Heo
2018-01-23 16:12 ` Sergey Senozhatsky
2018-01-23 16:13 ` Steven Rostedt
2018-01-23 17:21 ` Tejun Heo
2018-04-23 5:35 ` Sergey Senozhatsky
2018-01-23 16:01 ` Sergey Senozhatsky
2018-01-23 16:24 ` Steven Rostedt
2018-01-24 2:11 ` Sergey Senozhatsky
2018-01-24 2:52 ` Steven Rostedt
2018-01-24 4:44 ` Sergey Senozhatsky
2018-01-23 17:22 ` Tejun Heo
2018-01-20 12:19 ` Tejun Heo
2018-01-20 14:51 ` Steven Rostedt
2018-01-17 20:05 ` Tejun Heo
2018-01-18 5:43 ` Sergey Senozhatsky
2018-01-18 11:51 ` Petr Mladek
2018-01-18 5:42 ` Sergey Senozhatsky
2018-01-12 3:12 ` Sergey Senozhatsky
2018-01-12 2:56 ` Sergey Senozhatsky
2018-01-12 3:21 ` Steven Rostedt
2018-01-12 10:05 ` Sergey Senozhatsky
2018-01-12 12:21 ` Steven Rostedt
2018-01-12 12:55 ` Petr Mladek [this message]
2018-01-13 7:31 ` Sergey Senozhatsky
2018-01-15 8:51 ` Petr Mladek
2018-01-15 9:48 ` Sergey Senozhatsky
2018-01-16 5:16 ` Sergey Senozhatsky
2018-01-16 9:08 ` Petr Mladek
2018-01-15 12:08 ` Steven Rostedt
2018-01-16 4:51 ` Sergey Senozhatsky
2018-01-13 7:28 ` Sergey Senozhatsky
2018-01-15 10:17 ` Petr Mladek
2018-01-15 11:50 ` Petr Mladek
2018-01-16 6:10 ` Sergey Senozhatsky
2018-01-16 9:36 ` Petr Mladek
2018-01-16 10:10 ` Sergey Senozhatsky
2018-01-16 16:06 ` Steven Rostedt
2018-01-16 5:23 ` Sergey Senozhatsky
2018-01-15 12:06 ` Steven Rostedt
2018-01-15 14:45 ` Petr Mladek
2018-01-16 2:23 ` Sergey Senozhatsky
2018-01-16 4:47 ` Sergey Senozhatsky
2018-01-16 10:19 ` Petr Mladek
2018-01-17 2:24 ` Sergey Senozhatsky
2018-01-16 15:45 ` Steven Rostedt
2018-01-17 2:18 ` Sergey Senozhatsky
2018-01-17 13:04 ` Petr Mladek
2018-01-17 15:24 ` Steven Rostedt
2018-01-18 4:31 ` Sergey Senozhatsky
2018-01-18 15:22 ` Steven Rostedt
2018-01-16 10:13 ` Petr Mladek
2018-01-17 6:29 ` Sergey Senozhatsky
2018-01-16 1:46 ` Sergey Senozhatsky
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