From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Petr Mladek <pmladek@suse.com>, Tejun Heo <tj@kernel.org>,
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@rostedt.homelinux.com,
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: Wed, 24 Jan 2018 11:11:33 +0900 [thread overview]
Message-ID: <20180124021034.GA651@jagdpanzerIV> (raw)
In-Reply-To: <20180123112436.0c94bc2e@gandalf.local.home>
Hello,
On (01/23/18 11:24), Steven Rostedt wrote:
[..]
> > With WQ we don't lockup the kernel, because we flush printk_safe in
> > preemptible context. And people are very much expected to fix the
> > misbehaving consoles. But that should not be printk_safe problem.
>
> Right, but now you just made printk safe unreliable to get information
> out, because you need to wait for a schedule to occur, and if there's
> issues, like a deadlock, that thread will never run. And you just lost
> you lockdep splat.
Yes and No.
printk_safe and printk_nmi are unreliable - both need irq_work. That's
why we forcibly flush those buffers in panic(). At least for printk_safe
case, and I'm pretty sure the same stands for printk_nmi, we never said
that we will store all the messages that were printed from unsafe context
(recursion or NMI). The only thing we said - we will try not to deadlock
the system.
Now we are adding one more thing to printk_safe - we will also try not to
lockup the system.
Default printk_safe buffer size might not be enough to store a very large
lockdep splat. And we will report that the buffer is too small and that we
lost some of the lines: "here is what we have, we lost N lines, but at least
we didn't deadlock the system". See f975237b76827956fe13ecfe993a319158e2c303
for more details, it contains a list of recursive-printk deadlock scenarios
that printk_safe was meant to handle.
It is possible and OK to lose messages in printk_safe/printk_nmi
printk_safe_enter_irqsave()
printk
printk
...
...
printk
printk
printk_safe_exit_irqrestore()
No flush will take place as long as there is no IRQ on that CPU.
But printk_safe and printk_nmi are solving different problem in
the first place.
> > I'll re-read this one tomorrow. Not quite following it.
>
> I'll add more capitals next time ;-)
Ha-ha-ha ;)
[..]
> > pintk_safe was designed to be recursive. It was never designed to be
> > used to troubleshoot or debug consoles. But it was designed to be
> > recursive - because that's the sort of the problems it was meant to
> > handle: recursive printks that would otherwise deadlock us. That's why
> > we have it in the first place.
>
> So printk safe is only triggered when at the same context? If we can
> guarantee that printk safe is triggered only when its because a printk
> is happening at the same context (not because of an interrupt, but
> really at the same context, using my context check), then I'm fine with
> delaying them to a work queue.
printk_safe is for printk recursion only. It happens in the same context
only. When we switch to printk_safe we disable local IRQs, NMIs have their
own printk_nmi thing. And the way we flush printk_safe is mostly recursive.
Because we flush when we know that we will not deadlock [as much as we can;
we can't control any 3rd party locks which might be involved; thus
printk_deferred() usage].
Usually it's something like
printk
spin_lock_irqsave(logbuf_lock)
printk
spin_lock_irqsave(logbuf_lock) << deadlock
What we have with printk_safe is
printk
local_irq_save
printk_safe_enter
spin_lock(logbuf_lock)
printk
vprintk_safe
queue irq work
spin_unlock(logbuf_lock)
printk_safe_exit
local_irq_restore
>>> IRQ work
printk_safe_flush
printk
spin_lock_irqsave(logbuf_lock)
log_store()
spin_unlock_irqrestore(logbuf_lock)
So we flush printk_safe ASAP, which usually (unless originally we were
not in IRQ) means that the flush is recursive, but safe - we don't
deadlock.
> That is, if we have this:
>
> printk()
> console_lock()
> <interrupt>
> printk()
> add to log buffer
> <print irq printk too>
> console_unlock();
Right. This is what we have right now. Every time we enable local IRQs in
the console_unlock() printing loop - we flush printk_safe. And that's the
problem.
> printk()
> console_lock()
> <console does a printk>
> put in printk safe buffer
> trigger work queue
> console_unlock()
> <work queue>
> flush safe buffer
> printk()
Right. This is what we will have with WQ. We don't flush printk_safe until
we return from console_unlock(). Because printk() disables preemption for
the duration of console_unlock(), we can't schedule WQ on that CPU. And we
schedule flushing work only on the CPU that has triggered the recursion.
Another thing:
console_lock()
blah blah
console_unlock()
In this case we will flush printk_safe withing the printing loop.
Immediately. But we don't care - the CPU is preemptible, we don't
lock up the kernel.
> Then I'm fine with that.
>
> I have to look at the latest code. If this is indeed what we have, then
> I admit I misunderstood the problem you want to solve.
>
> I only want recursive printks (those that are actually triggered by
> doing a printk) to be allowed to be delayed.
>
> Make sense?
Please take a look.
-ss
next prev parent reply other threads:[~2018-01-24 2:11 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 [this message]
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
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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