Waiman Long <long...@redhat.com> writes: > On 02/27/2019 08:18 PM, Huang, Ying wrote: >> Waiman Long <long...@redhat.com> writes: >> >>> On 02/26/2019 12:30 PM, Linus Torvalds wrote: >>>> On Tue, Feb 26, 2019 at 12:17 AM Huang, Ying <ying.hu...@intel.com> 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 it is better to have i_mmap and the rwsem in separate cachelines. Right?
Yes. >> 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. > > Write to the rwsem's count mostly happens at lock and unlock times. It > is the constant spinning on owner by the optimistic waiter that is > likely to cause the most problem when its cacheline is shared with > another piece of data outside of the rwsem that is rewritten to fairly > frequently. Perhaps moving i_mmap further away from i_mmap_rwsem may help. Yes. I think rwsem->owner is more important too. rwsem->count has measurable effect too. And yes, moving i_mmap further away should help rwsem->count sharing too. Best Regards, Huang, Ying > Cheers, > Longman
