Re: [LKP] [page cache] eb797a8ee0: vm-scalability.throughput -16.5% regression

[Waiman Long <longman@redhat.com>]

On 02/26/2019 12:30 PM, Linus Torvalds wrote:
> On Tue, Feb 26, 2019 at 12:17 AM Huang, Ying <ying.huang@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