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

[Rafael J. Wysocki]

On Monday, July 11, 2016 03:46:01 PM Viresh Kumar wrote:
> On 12-07-16, 00:44, Rafael J. Wysocki wrote:
> > 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.
> 
> Yeah and I am not sure how should I go ahead about this issue now :)

FWIW, I think the reason why the "synchronous printk" works is because after
disabling the non-boot CPU, the only remaining one disables local interrupts
and won't do any async work any more until resume.

> > And *please* CC PM-related stuff to linux-pm.
> 
> Sure. I wasn't sure initially when this thread got started, that it is
> a PM related stuff and so didn't do it. As it was all about printk and
> hogging :)

But you started to talk about suspend/resume and such at one point and that
message should have been CCed to linux-pm.

And the reason why is because problems you see during suspend/resume may very
well be suspend-specific and not visible otherwise.  In which case you'll
likely need input from the people on linux-pm.

Thanks,
Rafael