Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount

[Waiman Long <waiman.long@hp.com>]

On 08/30/2013 03:40 PM, Al Viro wrote:
> On Fri, Aug 30, 2013 at 03:20:48PM -0400, Waiman Long wrote:
>
>> There are more contention in the lglock than I remember for the run
>> in 3.10. This is an area that I need to look at. In fact, lglock is
>> becoming a problem for really large machine with a lot of cores. We
>> have a prototype 16-socket machine with 240 cores under development.
>> The cost of doing a lg_global_lock will be very high in that type of
>> machine given that it is already high in this 80-core machine. I
>> have been thinking about instead of per-cpu spinlocks, we could
>> change the locking to per-node level. While there will be more
>> contention for lg_local_lock, the cost of doing a lg_global_lock
>> will be much lower and contention within the local die should not be
>> too bad. That will require either a per-node variable infrastructure
>> or simulated with the existing per-cpu subsystem.
> Speaking of lglock, there's a low-hanging fruit in that area: we have
> no reason whatsoever to put anything but regular files with FMODE_WRITE
> on the damn per-superblock list - the *only* thing it's used for is
> mark_files_ro(), which will skip everything except those.  And since
> read opens normally outnumber the writes quite a bit...  Could you
> try the diff below and see if it changes the picture?  files_lglock
> situation ought to get better...
>
>

Sure. I will try that out, but it probably won't help too much in this 
test case. The perf profile that I sent out in my previous mail is only 
partial. The actual one for lg_global_lock was:

      1.01%            reaim  [kernel.kallsyms]        [k] lg_global_lock
                       |
                       --- lg_global_lock
                           mntput_no_expire
                           mntput
                           __fput
                           ____fput
                           task_work_run
                           do_notify_resume
                           int_signal
                          |
                          |--51.62%-- __shmdt
                          |
                           --48.38%-- __shmctl

So it is the mnput_no_expire() function that is doing all the 
lg_global_lock() calls.

Regards,
Longman