Re: [RFC v2 17/18] rcu: Convert RCU gp kthreads into kthread worker API

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Oct 01, 2015 at 05:43:00PM +0200, Petr Mladek wrote:
> On Mon 2015-09-28 10:14:37, Paul E. McKenney wrote:
> > On Mon, Sep 21, 2015 at 03:03:58PM +0200, Petr Mladek wrote:
> > > Kthreads are currently implemented as an infinite loop. Each
> > > has its own variant of checks for terminating, freezing,
> > > awakening. In many cases it is unclear to say in which state
> > > it is and sometimes it is done a wrong way.
> > > 
> > > The plan is to convert kthreads into kthread_worker or workqueues
> > > API. It allows to split the functionality into separate operations.
> > > It helps to make a better structure. Also it defines a clean state
> > > where no locks are taken, IRQs blocked, the kthread might sleep
> > > or even be safely migrated.
> > > 
> > > The kthread worker API is useful when we want to have a dedicated
> > > single kthread for the work. It helps to make sure that it is
> > > available when needed. Also it allows a better control, e.g.
> > > define a scheduling priority.
> > > 
> > > This patch converts RCU gp threads into the kthread worker API.
> > > They modify the scheduling, have their own logic to bind the process.
> > > They provide functions that are critical for the system to work
> > > and thus deserve a dedicated kthread.
> > > 
> > > This patch tries to split start of the grace period and the quiescent
> > > state handling into separate works. The motivation is to avoid
> > > wait_events inside the work. Instead it queues the works when
> > > appropriate which is more typical for this API.
> > > 
> > > On one hand, it should reduce spurious wakeups where the condition
> > > in the wait_event failed and the kthread went to sleep again.
> > > 
> > > On the other hand, there is a small race window when the other
> > > work might get queued. We could detect and fix this situation
> > > at the beginning of the work but it is a bit ugly.
> > > 
> > > The patch renames the functions kthread_wake() to kthread_worker_poke()
> > > that sounds more appropriate.
> > > 
> > > Otherwise, the logic should stay the same. I did a lot of torturing
> > > and I did not see any problem with the current patch. But of course,
> > > it would deserve much more testing and reviewing before applying.
> > 
> > Suppose I later need to add helper kthreads to parallelize grace-period
> > initialization.  How would I implement that in a freeze-friendly way?
> 
> I have been convinced that there only few kthreads that really need
> freezing. See the discussion around my first attempt at
> https://lkml.org/lkml/2015/6/13/190
> 
> In fact, RCU is a good example of kthreads that should not get
> frozen because they are needed even later when the system
> is suspended.
> 
> If I understand it correctly, they will do the job until most devices
> and all non-boot CPUs are disabled. Then the task doing the suspend
> will get scheduled. It will write the image and stop the machine.
> RCU should not be needed by this very last step.
> 
> By other words. RCU should not be much concerned about freezing.
> 
> If you are concerned about adding more kthreads, it should be
> possible to just add more workers if we agree on using the
> kthreads worker API.

OK, I will bite.  If RCU should not be much concerned about freezing,
why can't it retain its current simpler implementation using the current
kthread APIs?

							Thanx, Paul

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