On Sun, Nov 15, 2020 at 12:53:39PM -0800, Paul E. McKenney wrote: > On Sun, Nov 15, 2020 at 03:35:51PM -0500, Kent Overstreet wrote: > > Yeah. My one quibble is, instead of having to call poll_state_synchronize_srcu() > > on every cookie - would it be possible to get function that returns a cookie we > > can compare against (i.e. with time_after())? I'm going to be use this in the > > shrinker where we have to walk and check potentially tens of thousands of > > objects. > > If the cookies compare equal, poll_state_synchronize_srcu() will treat > them the same. If that does not help, could you please show me a code > snippet illustrating what you would like to do? > > (Yes, even if equality comparison works, I probably need to give you > an API member just in case the nature of grace-period sequence numbers > changes in the future.) Having just looked at your code - I think it's fine as is. I wouldn't complain if it was a static inline, though. > > > > Oh, and due to historical inertia, Tiny SRCU's grace-period sequence > > > number is only 16 bits. I can change this easily, but I need to know > > > that it is a real problem for you before I can do so. > > > > > > The potential problem for you is that if you let a given cookie lie > > > dormant for 16384 grace periods, it will take another 16385 grace > > > periods for get_state_synchronize_srcu() to say that a grace period > > > has elapsed. > > > > > > In contrast, Tree SRCU's grace-period sequence number is either 32 bits > > > or 64 bits, depending on the size of unsized long. > > > > It's not something I'd lose sleep over, but I think it could be. If there isn't > > memory pressure, then the shrinker won't be running and we won't be freeing the > > objects with the oldest cookies, but freeing them internally will be creating > > new grace periods - but if I make sure we're reusing objects in LIFO order would > > also work against the shrinker actually being able to free any objects, so not > > sure I want to do that... > > OK, I will leave it, at least until you tell me otherwise. > > I probably need to add a warning to the header comment, though... > > Thanx, Paul Thanks!
