Re: [lkp-robot] [fs/locks] 9d21d181d0: will-it-scale.per_process_ops -14.1% regression

["Benjamin Coddington" <bcodding@redhat.com>]

On 1 Jun 2017, at 11:48, Jeff Layton wrote:

> On Thu, 2017-06-01 at 11:14 -0400, J. Bruce Fields wrote:
>> On Thu, Jun 01, 2017 at 08:59:21AM -0400, Jeff Layton wrote:
>>> I'm not so sure. That would only be the case if the thing were 
>>> marked
>>> for manadatory locking (a really rare thing).
>>>
>>> The test is really simple and I don't think any read/write activity 
>>> is
>>> involved:
>>>
>>>     https://github.com/antonblanchard/will-it-scale/blob/master/tests/lock1.c
>>
>> So it's just F_WRLCK/F_UNLCK in a loop spread across multiple cores?
>> I'd think real workloads do some work while holding the lock, and a 
>> 15%
>> regression on just the pure lock/unlock loop might not matter?  But 
>> best
>> to be careful, I guess.
>>
>> --b.
>>
>
> Yeah, that's my take.
>
> I was assuming that getting a pid reference would be essentially free,
> but it doesn't seem to be.
>
> So, I think we probably want to avoid taking it for a file_lock that 
> we
> use to request a lock, but do take it for a file_lock that is used to
> record a lock. How best to code that up, I'm not quite sure...

Maybe as simple as only setting fl_nspid in locks_insert_lock_ctx(), but
that seems to just take us back to the problem of getting the pid wrong 
if
the lock is inserted later by a different worker than created the 
request.

I have a mind now to just drop fl_nspid off the struct file_lock 
completely,
and instead just carry fl_pid, and when we do F_GETLK, we can do:

task = find_task_by_pid_ns(fl_pid, init_pid_ns)
fl_nspid = task_pid_nr_ns(task, task_active_pid_ns(current))

That moves all the work off into the F_GETLK case, which I think is not 
used
so much.

Ben