From: Petr Mladek <pmladek@suse.com>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Qian Cai <cai@lca.pw>, Steven Rostedt <rostedt@goodmis.org>,
Michal Hocko <mhocko@kernel.org>,
Eric Dumazet <eric.dumazet@gmail.com>,
davem@davemloft.net, netdev@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] net/skbuff: silence warnings under memory pressure
Date: Wed, 20 Nov 2019 17:13:34 +0100 [thread overview]
Message-ID: <20191120161334.p63723g4jyk6k7p3@pathway.suse.cz> (raw)
In-Reply-To: <20191120013005.GA3191@tigerII.localdomain>
On Wed 2019-11-20 10:30:05, Sergey Senozhatsky wrote:
> On (19/11/19 10:41), Petr Mladek wrote:
> [..]
> > > > I do not like this. As a result, normal printk() will always deadlock
> > > > in the scheduler code, including WARN() calls. The chance of the
> > > > deadlock is small now. It happens only when there is another
> > > > process waiting for console_sem.
> > >
> > > Why would it *always* deadlock? If this is the case, why we don't *always*
> > > deadlock doing the very same wake_up_process() from console_unlock()?
> >
> > I speak about _normal_ printk() and not about printk_deferred().
> >
> > wake_up_process() is called in console_unlock() only when
> > sem->wait_list is not empty, see up() in kernel/locking/semaphore.c.
> > printk() itself uses console_trylock() and does not wait.
>
> > I believe that this is the rason why printk_sched() was added
> > so late in 2012.
>
> Right. I also think scheduler people do pretty nice work avoiding printk
> calls under ->rq lock.
>
> What I tried to say - it's really not that hard to have a non-empty
> console_sem ->wait_list, any "wrong" printk() call from scheduler
> will deadlock us, because we have something to wake_up().
I am sorry but I do not take this as an argument that it would be
acceptable to replace irq_work_queue() with wake_up_interruptible().
It is the first time that I hear about problem caused by the
irq_work(). But we deal with deadlocks caused by wake_up() for years.
It would be like replacing a lightly dripping tap with a heavily
dripping one.
I see reports with WARN() from scheduler code from time to time.
I would get reports about silent death instead.
RT guys are going to make printk() fully lockless. It would be
really great achievement. irq_work is lockless. While wake_up()
is not.
There must be a better way how to break the infinite loop caused
by the irq_work.
Best Regards,
Petr
next prev parent reply other threads:[~2019-11-20 16:13 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-30 14:57 [PATCH] net/skbuff: silence warnings under memory pressure Qian Cai
2019-08-30 15:11 ` Eric Dumazet
2019-08-30 15:25 ` Qian Cai
2019-08-30 16:15 ` Eric Dumazet
2019-08-30 18:06 ` Qian Cai
2019-09-03 13:22 ` Michal Hocko
2019-09-03 15:42 ` Qian Cai
2019-09-03 18:53 ` Michal Hocko
2019-09-03 21:42 ` Qian Cai
2019-09-04 6:15 ` Michal Hocko
2019-09-04 6:41 ` Sergey Senozhatsky
2019-09-04 6:54 ` Michal Hocko
2019-09-04 7:19 ` Sergey Senozhatsky
2019-09-04 7:43 ` Sergey Senozhatsky
2019-09-04 12:14 ` Qian Cai
2019-09-04 14:48 ` Sergey Senozhatsky
2019-09-04 15:07 ` Qian Cai
2019-09-04 20:42 ` Qian Cai
2019-09-05 8:32 ` Eric Dumazet
2019-09-05 14:09 ` Qian Cai
2019-09-05 15:06 ` Eric Dumazet
2019-09-05 15:14 ` Eric Dumazet
2019-09-05 11:32 ` Sergey Senozhatsky
2019-09-05 16:03 ` Qian Cai
2019-09-05 17:14 ` Steven Rostedt
2019-09-06 2:50 ` Sergey Senozhatsky
2019-09-06 4:32 ` Sergey Senozhatsky
2019-09-06 21:17 ` Qian Cai
2019-09-05 17:23 ` Steven Rostedt
2019-09-06 3:39 ` Sergey Senozhatsky
2019-09-06 15:32 ` Petr Mladek
2019-09-09 1:10 ` Sergey Senozhatsky
2019-09-06 14:55 ` Petr Mladek
2019-09-06 19:51 ` Sergey Senozhatsky
2019-11-14 17:12 ` Qian Cai
2019-11-18 15:27 ` Petr Mladek
2019-11-19 0:41 ` Sergey Senozhatsky
2019-11-19 9:41 ` Petr Mladek
2019-11-19 15:58 ` Qian Cai
2019-11-20 1:30 ` Sergey Senozhatsky
2019-11-20 16:13 ` Petr Mladek [this message]
2019-11-21 1:05 ` Sergey Senozhatsky
2019-11-21 9:15 ` Petr Mladek
2019-09-04 7:00 ` Sergey Senozhatsky
2019-09-04 8:25 ` Michal Hocko
2019-09-04 11:59 ` Qian Cai
2019-09-04 12:07 ` Michal Hocko
2019-09-04 12:28 ` Qian Cai
2019-09-07 11:00 ` Tetsuo Handa
2019-09-04 6:15 ` Michal Hocko
2019-09-02 14:24 ` Vlastimil Babka
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