> On Apr 29, 2021, at 9:12 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Wed, Apr 28, 2021 at 05:05:17PM -0700, Andy Lutomirski wrote: >>> On Wed, Apr 28, 2021 at 5:02 PM Michel Lespinasse <michel@xxxxxxxxxxxxxx> wrote: >>> Thanks Paul for confirming / clarifying this. BTW, it would be good to >>> add this to the rcu header files, just so people have something to >>> reference to when they depend on such behavior (like fast GUP >>> currently does). >> >> Or, even better, fast GUP could add an explicit RCU read lock. >> >>> >>> Going back to my patch. I don't need to protect against THP splitting >>> here, as I'm only handling the small page case. So when >>> MMU_GATHER_RCU_TABLE_FREE is enabled, I *think* I could get away with >>> using only an rcu read lock, instead of disabling interrupts which >>> implicitly creates the rcu read lock. I'm not sure which way to go - >>> fast GUP always disables interrupts regardless of the >>> MMU_GATHER_RCU_TABLE_FREE setting, and I think there is a case to be >>> made for following the fast GUP stes rather than trying to be smarter. >> >> How about adding some little helpers: >> >> lockless_page_walk_begin(); >> >> lockless_page_walk_end(); >> >> these turn into RCU read locks if MMU_GATHER_RCU_TABLE_FREE and into >> irqsave otherwise. And they're somewhat self-documenting. > > One of the worst things we can do while holding a spinlock is take a > cache miss because we then delay for several thousand cycles to wait for > the cache line. That gives every other CPU a really long opportunity > to slam into the spinlock and things go downhill fast at that point. > We've even seen patches to do things like read A, take lock L, then read > A to avoid the cache miss while holding the lock. > > What sort of performance effect would it have to free page tables > under RCU for all architectures? It's painful on s390 & powerpc because > different tables share the same struct page, but I have to believe that's > a solvable problem. The IPI locking mechanism is entirely useless on any architecture that wants to do paravirt shootdowns, so this seems like a good strategy to me.
