On Fri, Aug 12, 2011 at 10:41:36AM -0700, Paul E. McKenney wrote: > On Fri, Aug 12, 2011 at 06:57:49PM +0200, Johannes Weiner wrote: > > On Fri, Aug 12, 2011 at 09:08:13AM -0700, Paul E. McKenney wrote: > > > On Fri, Aug 12, 2011 at 05:36:16PM +0200, Andrea Arcangeli wrote: > > > > On Tue, Aug 09, 2011 at 04:04:21AM -0700, Michel Lespinasse wrote: > > > > > - Use my proposed page count lock in order to avoid the race. One > > > > > would have to convert all get_page_unless_zero() sites to use it. I > > > > > expect the cost would be low but still measurable. > > > > > > > > I didn't yet focus at your problem after we talked about it at MM > > > > summit, but I seem to recall I suggested there to just get to the head > > > > page and always take the lock on it. split_huge_page only works at 2M > > > > aligned pages, the rest you don't care about. Getting to the head page > > > > compound_lock should be always safe. And that will still scale > > > > incredibly better than taking the lru_lock for the whole zone (which > > > > would also work). And it seems the best way to stop split_huge_page > > > > without having to alter the put_page fast path when it works on head > > > > pages (the only thing that gets into put_page complex slow path is the > > > > release of tail pages after get_user_pages* so it'd be nice if > > > > put_page fast path still didn't need to take locks). > > > > > > > > > - It'd be sweet if one could somehow record the time a THP page was > > > > > created, and wait for at least one RCU grace period *starting from the > > > > > recorded THP creation time* before splitting huge pages. In practice, > > > > > we would be very unlikely to have to wait since the grace period would > > > > > be already expired. However, I don't think RCU currently provides such > > > > > a mechanism - Paul, is this something that would seem easy to > > > > > implement or not ? > > > > > > It should not be hard. I already have an API for rcutorture testing > > > use, but it is not appropriate for your use because it is unsynchronized. > > > > > > We need to be careful with what I give you and how you interpret it. > > > The most effective approach would be for me to give you an API that > > > filled in a cookie given a pointer to one, then another API that took > > > pointers to a pair of cookies and returned saying whether or not a > > > grace period had elapsed. You would do something like the following: > > > > > > rcu_get_gp_cookie(&pagep->rcucookie); > > > . . . > > > > > > rcu_get_gp_cookie(&autovarcookie); > > > if (!rcu_cookie_gp_elapsed(&pagep->rcucookie, &autovarcookie)) > > > synchronize_rcu(); > > > > > > So, how much space do I get for ->rcucookie? By default, it is a pair > > > of unsigned longs, but I could live with as small as a single byte if > > > you didn't mind a high probability of false negatives (me telling you > > > to do a grace period despite 16 of them having happened in the meantime > > > due to overflow of a 4-bit field in the byte). > > > > > > That covers TREE_RCU and TREE_PREEMPT_RCU, on to TINY_RCU and TINY_PREEMPT_RCU. > > > > > > TINY_RCU will require more thought, as it doesn't bother counting grace > > > periods. Ah, but in TINY_RCU, synchronize_rcu() is free, so I simply > > > make rcu_cookie_gp_elapsed() always return false. > > > > > > OK, TINY_PREEMPT_RCU... It doesn't count grace periods, either. But it > > > is able to reliably detect if there are any RCU readers in flight, > > > and there normally won't be, so synchronize_rcu() is again free in the > > > common case. And no, I don't want to count grace periods as this would > > > increase the memory footprint. And the whole point of TINY_PREEMPT_RCU > > > is to be tiny, after all. ;-) > > > > I understand you want to be careful with the promises you make in the > > API. How about not even exposing the check for whether a grace period > > elapsed, but instead provide a specialized synchronize_rcu()? > > > > Something like > > > > void synchronize_rcu_with(rcu_time_t time) > > > > that only promises all readers from the specified time are finished. > > > > [ And synchronize_rcu() would be equivalent to > > synchronize_rcu_with(rcu_current_time()) if I am not mistaken. ] > > > > Then you wouldn't need to worry about how the return value of > > rcu_cookie_gp_elapsed() might be interpreted, could freely implement > > it equal to synchronize_rcu() on TINY_RCU, the false positives with > > small cookies would not be about correctness but merely performance. > > > > And it should still be all that which the THP case requires. > > > > Would that work? > > I currently don't record the times at which past grace periods start > and finish, but you can think of the cookie I was proposing as being a > specialized timestamp that measures the passage of time in terms of the > number of grace periods that have started and finished. ;-) Oh, absolutely, that is what I meant. Sorry if it wasn't clear. My proposal was more about the function interface, not the unit of the cookies that are passed around. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>