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? -- 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>