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. > > > > > > Thanx, Paul > > > > 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? > > Thanx, Paul 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. -- MST
