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

[Waiman Long]

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
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