On Mon, Aug 19, 2024 at 12:16:42AM -0400, Kent Overstreet wrote: > On Sun, Aug 18, 2024 at 08:52:59PM GMT, Paul E. McKenney wrote: > > On Sun, Aug 18, 2024 at 09:19:16PM -0400, Kent Overstreet wrote: > > > On Sun, Aug 18, 2024 at 10:01:42AM GMT, Paul E. McKenney wrote: > > > > But you only need one callback per live outstanding "cookie" returned > > > > from get_state_synchronize_rcu*() or start_poll_synchronize_rcu(). > > > > Or am I missing something here? > > > > > > Maybe I am? > > > > > > I've been assuming that if rcu callbacks are getting punted off to a > > > kthread that we can't rely on them being completed in any particular > > > timeframe - i.e. the number of grace periods with outstanding callbacks > > > would be unbounded. > > > > Exactly, and that is why there is a check for expired cookies during the > > addition of a new data element that needs to wait for a grace period. > > The only purpose of that call_rcu() callback is to act as a fallback in > > case there is a long period of time with no newly arriving data elements > > that need to wait for a grace period. > > > > > You're saying that NUM_ACTIVE_RCU_POLL_FULL_OLDSTATE _does_ include > > > grace periods with outstanding callbacks? Just want to be clear on that. > > > > No, it does not, but given the algorithm outlined in my previous > > email, it doesn't need to. The point of the callbacks is *not* to > > unconditionally drive things forward, but instead to provide a fallback > > to do the necessary processing in the case nothing new arrives for an > > extended period of time. > > You can't use a fixed number of callback heads if there's going to be an > unbounded number of callback heads outstanding. The number of rcu_head structures is your choice, based on your choice of data structure. You can for example link together data elements that have the same value of get_state_synchronize_rcu() cookie, and use a single rcu_head structure for that group. You could then do whatever you want for the linking. But even if you do choose to have a large number of rcu_head structures, perhaps one per data element, there is no law saying that each and every one of them needs to be passed to call_rcu(). For example, kfree_rcu() requires an rcu_head structure in the objects passed to it, but in the common (non-OOM) case, those structures go unused in favor of pages of pointers. So what were you really trying to get across to me here? Thanx, Paul
