On Mon, Jul 22, 2019 at 12:32:17PM -0400, Michael S. Tsirkin wrote: > On Mon, Jul 22, 2019 at 09:25:51AM -0700, Paul E. McKenney wrote: > > On Mon, Jul 22, 2019 at 12:13:40PM -0400, Michael S. Tsirkin wrote: > > > On Mon, Jul 22, 2019 at 08:55:34AM -0700, Paul E. McKenney wrote: > > > > On Mon, Jul 22, 2019 at 11:47:24AM -0400, Michael S. Tsirkin wrote: > > > > > On Mon, Jul 22, 2019 at 11:14:39AM -0400, Joel Fernandes wrote: > > > > > > [snip] > > > > > > > > Would it make sense to have call_rcu() check to see if there are many > > > > > > > > outstanding requests on this CPU and if so process them before returning? > > > > > > > > That would ensure that frequent callers usually ended up doing their > > > > > > > > own processing. > > > > > > > > > > > > Other than what Paul already mentioned about deadlocks, I am not sure if this > > > > > > would even work for all cases since call_rcu() has to wait for a grace > > > > > > period. > > > > > > > > > > > > So, if the number of outstanding requests are higher than a certain amount, > > > > > > then you *still* have to wait for some RCU configurations for the grace > > > > > > period duration and cannot just execute the callback in-line. Did I miss > > > > > > something? > > > > > > > > > > > > Can waiting in-line for a grace period duration be tolerated in the vhost case? > > > > > > > > > > > > thanks, > > > > > > > > > > > > - Joel > > > > > > > > > > No, but it has many other ways to recover (try again later, drop a > > > > > packet, use a slower copy to/from user). > > > > > > > > True enough! And your idea of taking recovery action based on the number > > > > of callbacks seems like a good one while we are getting RCU's callback > > > > scheduling improved. > > > > > > > > By the way, was this a real problem that you could make happen on real > > > > hardware? > > > > > > > If not, I would suggest just letting RCU get improved over > > > > the next couple of releases. > > > > > > So basically use kfree_rcu but add a comment saying e.g. "WARNING: > > > in the future callers of kfree_rcu might need to check that > > > not too many callbacks get queued. In that case, we can > > > disable the optimization, or recover in some other way. > > > Watch this space." > > > > That sounds fair. > > > > > > If it is something that you actually made happen, please let me know > > > > what (if anything) you need from me for your callback-counting EBUSY > > > > scheme. > > > > > > If you mean kfree_rcu causing OOM then no, it's all theoretical. > > > If you mean synchronize_rcu stalling to the point where guest will OOPs, > > > then yes, that's not too hard to trigger. > > > > Is synchronize_rcu() being stalled by the userspace loop that is invoking > > your ioctl that does kfree_rcu()? Or instead by the resulting callback > > invocation? > > Sorry, let me clarify. We currently have synchronize_rcu in a userspace > loop. I have a patch replacing that with kfree_rcu. This isn't the > first time synchronize_rcu is stalling a VM for a long while so I didn't > investigate further. Ah, so a bunch of synchronize_rcu() calls within a single system call inside the host is stalling the guest, correct? If so, one straightforward approach is to do an rcu_barrier() every (say) 1000 kfree_rcu() calls within that loop in the system call. This will decrease the overhead by almost a factor of 1000 compared to a synchronize_rcu() on each trip through that loop, and will prevent callback overload. Or if the situation is different (for example, the guest does a long sequence of system calls, each of which does a single kfree_rcu() or some such), please let me know what the situation is. Thanx, Paul
