Re: [PATCH 1/2] sched/wait: Break up long wake list walk

[Tim Chen]

On 08/18/2017 07:46 AM, Mel Gorman wrote:
> On Fri, Aug 18, 2017 at 02:20:38PM +0000, Liang, Kan wrote:
>>> Nothing fancy other than needing a comment if it works.
>>>
>>
>> No, the patch doesn't work.
>>
> 
> That indicates that it may be a hot page and it's possible that the page is
> locked for a short time but waiters accumulate.  What happens if you leave
> NUMA balancing enabled but disable THP?  Waiting on migration entries also
> uses wait_on_page_locked so it would be interesting to know if the problem
> is specific to THP.
> 
> Can you tell me what this workload is doing? I want to see if it's something
> like many threads pounding on a limited number of pages very quickly. If

It is a customer workload so we have limited visibility.  But we believe there are
some pages that are frequently accessed by all threads.

> it's many threads working on private data, it would also be important to
> know how each buffers threads are aligned, particularly if the buffers
> are smaller than a THP or base page size. For example, if each thread is
> operating on a base page sized buffer then disabling THP would side-step
> the problem but THP would be false sharing between multiple threads.
> 

Still, I don't think this problem is THP specific.  If there is a hot
regular page getting migrated, we'll also see many threads get
queued up quickly.  THP may have made the problem worse as migrating
it takes a longer time, meaning more threads could get queued up.

Thanks.

Tim