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. -Longman
