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 -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
