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? Thanx, Paul
