On Mon, Jul 06, 2020 at 02:06:45PM -0700, Paul E. McKenney wrote: > On Thu, Jul 02, 2020 at 10:19:08PM +0200, Sebastian Andrzej Siewior wrote: > > On 2020-07-02 09:48:26 [-0700], Paul E. McKenney wrote: > > > On Thu, Jul 02, 2020 at 04:12:16PM +0200, Sebastian Andrzej Siewior wrote: > > > > On 2020-06-30 11:35:34 [-0700], Paul E. McKenney wrote: > > > > > > This is not going to work together with the "wait context validator" > > > > > > (CONFIG_PROVE_RAW_LOCK_NESTING). As of -rc3 it should complain about > > > > > > printk() which is why it is still disabled by default. > > > > > > > > > > Fixing that should be "interesting". In particular, RCU CPU stall > > > > > warnings rely on the raw spin lock to reduce false positives due > > > > > to race conditions. Some thought will be required here. > > > > > > > > I don't get this part. Can you explain/give me an example where to look > > > > at? > > > > > > Starting from the scheduler-clock interrupt's call into RCU, > > > we have rcu_sched_clock_irq() which calls rcu_pending() which > > > calls check_cpu_stall() which calls either print_cpu_stall() or > > > print_other_cpu_stall(), depending on whether the stall is happening on > > > the current CPU or on some other CPU, respectively. > > > > > > Both of these last functions acquire the rcu_node structure's raw ->lock > > > and expect to do printk()s while holding it. > > > > … > > > Thoughts? > > > > Okay. So in the RT queue there is a printk() rewrite which fixes this > > kind of things. Upstream the printk() interface is still broken in this > > regard and therefore CONFIG_PROVE_RAW_LOCK_NESTING is disabled. > > [Earlier the workqueue would also trigger a warning but this has been > > fixed as of v5.8-rc1.] > > This was just me explaining why this bad, what debug function would > > report it and why it is not enabled by default. > > Whew!!! ;-) > > > > > > > So assume that this is fixed and enabled then on !PREEMPT_RT it will > > > > > > complain that you have a raw_spinlock_t acquired (the one from patch > > > > > > 02/17) and attempt to acquire a spinlock_t in the memory allocator. > > > > > > > > > > Given that the slab allocator doesn't acquire any locks until it gets > > > > > a fair way in, wouldn't it make sense to allow a "shallow" allocation > > > > > while a raw spinlock is held? This would require yet another GFP_ flag, > > > > > but that won't make all that much of a difference in the total. ;-) > > > > > > > > That would be one way of dealing with. But we could go back to > > > > spinlock_t and keep the memory allocation even for RT as is. I don't see > > > > a downside of this. And we would worry about kfree_rcu() from real > > > > IRQ-off region once we get to it. > > > > > > Once we get to it, your thought would be to do per-CPU queuing of > > > memory from IRQ-off kfree_rcu(), and have IRQ work or some such clean > > > up after it? Or did you have some other trick in mind? > > > > So for now I would very much like to revert the raw_spinlock_t back to > > the spinlock_t and add a migrate_disable() just avoid the tiny > > possible migration between obtaining the CPU-ptr and acquiring the lock > > (I think Joel was afraid of performance hit). > > Performance is indeed a concern here. > > > Should we get to a *real* use case where someone must invoke kfree_rcu() > > from a hard-IRQ-off region then we can think what makes sense. per-CPU > > queues and IRQ-work would be one way of dealing with it. > > It looks like workqueues can also be used, at least in their current > form. And timers. > > Vlad, Joel, thoughts? > Some high level thoughts: Currently everything is done in workqueue context, it means all freeing happens there. For RT kernel we can invoke a page allocator only for single kfree_rcu() argument(though we skip it). As for double one, it is impossible, that is why a simple path is used by linking rcu_head among each other for further reclaim in wq context. As of now, for RT, everything is already deferred. If we revert to spinlock_t then calling of kfree_rcu() from hard IRQ context is broken, even though we think that for RT kernel it will never happen. Therefore i do not see a clear motivation and benefits why we should revert to spinlock_t. IMHO, if we can avoid of such drawback i would go with that way, i.e. i would not like to think what to do with that when it becomes broken. Thanks! -- Vlad Rezki
