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SubjectRe: [LKP] [page cache] eb797a8ee0: vm-scalability.throughput -16.5% regression
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Waiman Long <longman@redhat.com> writes:

> On 02/27/2019 08:18 PM, Huang, Ying wrote:
>> Waiman Long <longman@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.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.
>> 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

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