linux-kernel - Re: [Query] Preemption (hogging) of the work handler

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Date:	Tue, 12 Jul 2016 00:44:08 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Jan Kara <jack@...e.cz>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Tejun Heo <tj@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	vlevenetz@...sol.com, vaibhav.hiremath@...aro.org,
	alex.elder@...aro.org, johan@...nel.org, akpm@...ux-foundation.org,
	rostedt@...dmis.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [Query] Preemption (hogging) of the work handler

On Monday, July 11, 2016 03:35:01 PM Viresh Kumar wrote:
> Hi Sergey and Jan,
> 
> On 12-07-16, 00:44, Sergey Senozhatsky wrote:
> > right. apart from cases when the existing console_unlock() behaviour can
> > simply "block" a process to flush the log_buf to slow serial consoles
> > (regardless the  process execution context) and make the system less
> > responsive, I have around ~10 absolutely different scenarios on my list that
> > may cause soft/hard lockups, rcu stalls, oom-s, etc. and console_unlock() is
> > the root cause there. the simplest ones involve heavy printk() usage, the
> > trickier ones do not necessarily have anything that is abusing printk(): a
> > moderate printk() pressure coming from other CPUs on the system and more or
> > less active tty -> UART can do the trick, because uart interrupt service
> > routine and call_console_drivers()->write() have to compete for the same
> > uart port spin_lock. soft lockups are probably the most common problems,
> > though, it's not all that easy to catch, because watchdog does not ring
> > the bell straight after preempt_enable(), but from hrtimer interrupt, that
> > happens approx every 4 seconds. by this time CPU can be somewhere far away
> > from console_unlock(). I had an idea of doing watchdog soft lockup check
> > from preempt_enable(), when it brings preempt_count down to zero, but not
> > sure I can recall how well did it go.
> 
> Thanks for your feedback guys, and I have one more blocking issue
> where I need your help/advice.
> 
> So, the excess printing in our case is done in parallel to system
> suspend. And that can very much happen after all the non-boot CPUs are
> offlined.
> 
> Sometimes, the platform doesn't come back after suspend. I have tried
> enabling no-console-suspend and the last line it prints is:
> 
>         Disabling non-boot CPUs
> 
> And nothing after that at all. We have to forcefully reboot the phone
> after that. Moving the prints to they synchronous way (using
> echo 1 > /sys/module/printk/parameters/synchronous), fixes that issue.

But no_console_suspend is best-effort by design.

And *please* CC PM-related stuff to linux-pm.

Thanks,
Rafael