Waiman Long <[email protected]> writes: > On 02/26/2019 12:30 PM, Linus Torvalds wrote: >> On Tue, Feb 26, 2019 at 12:17 AM Huang, Ying <[email protected]> wrote: >>> As for fixing. Should we care about the cache line alignment of struct >>> inode? Or its size is considered more important because there may be a >>> huge number of struct inode in the system? >> Thanks for the great analysis. >> >> I suspect we _would_ like to make sure inodes are as small as >> possible, since they are everywhere. Also, they are usually embedded >> in other structures (ie "struct inode" is embedded into "struct >> ext4_inode_info"), and unless we force alignment (and thus possibly >> lots of padding), the actual alignment of 'struct inode' will vary >> depending on filesystem. >> >> So I would suggest we *not* do cacheline alignment, because it will >> result in random padding. >> >> But it sounds like maybe the solution is to make sure that the >> different fields of the inode can and should be packed differently? >> >> So one thing to look at is to see what fields in 'struct inode' might >> be best moved together, to minimize cache accesses. >> >> And in particular, if this is *only* an issue of "struct >> rw_semaphore", maybe we should look at the layout of *that*. In >> particular, I'm getting the feeling that we should put the "owner" >> field right next to the "count" field, because the normal >> non-contended path only touches those two fields. > > That is true. Putting the two next to each other reduces the chance of > needing to touch 2 cachelines to acquire a rwsem. > >> Right now those two fields are pretty far from each other in 'struct >> rw_semaphore', which then makes the "oops they got allocated in >> different cachelines" much more likely. >> >> So even if 'struct inode' layout itself isn't changed, maybe just >> optimizing the layout of 'struct rw_semaphore' a bit for the common >> case might fix it all up. >> >> Waiman, I didn't check if your rewrite already possibly fixes this? > > My current patch doesn't move the owner field, but I will add one to do > it. That change alone probably won't solve the regression we see here. > The optimistic spinner is spinning on the on_cpu flag of the task > structure as well as the rwsem->owner value (looking for change). The > lock holder only need to touch the count/owner values once at unlock. > However, if other hot data variables are in the same cacheline as > rwsem->owner, we will have cacaheline bouncing problem. So we need to > pad some rarely touched variables right before the rwsem in order to > reduce the chance of false cacheline sharing.
Yes. And if my understanding were correct, if the rwsem is locked, the new rw_sem users (which calls down_write()) will write rwsem->count and some other fields of rwsem. This will cause cache ping-pong between lock holder and the new users too if the memory accessed by lock holder shares the same cache line with rwsem->count, thus hurt the system performance. For the regression reported, the rwsem holder will change address_space->i_mmap, if I put i_mmap and rwsem->count in the same cache line and rwsem->owner in a different cache line, the performance can improve ~8.3%. While if I put i_mmap in one cache line and all fields of rwsem in another different cache line, the performance can improve ~12.9% (in another machine, where the regression is ~14%). So I think in the heavily contended situation, we should put the fields accessed by rwsem holder in a different cache line of rwsem. But in un-contended situation, we should put the fields accessed in rwsem holder and rwsem in the same cache line to reduce the cache footprint. The requirement of un-contended and heavily contended situation is contradicted. Best Regards, Huang, Ying > -Longman
