linux-kernel - Re: [RFC][PATCH 00/13] percpu rwsem -v2

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Date:	Tue, 23 Jun 2015 21:36:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Daniel Wagner <daniel.wagner@...-carit.de>
Cc:	oleg@...hat.com, paulmck@...ux.vnet.ibm.com, tj@...nel.org,
	mingo@...hat.com, linux-kernel@...r.kernel.org, der.herr@...r.at,
	dave@...olabs.net, riel@...hat.com, viro@...IV.linux.org.uk,
	torvalds@...ux-foundation.org, jlayton@...chiereds.net
Subject: Re: [RFC][PATCH 00/13] percpu rwsem -v2

On Tue, Jun 23, 2015 at 07:50:12PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 23, 2015 at 04:56:39PM +0200, Daniel Wagner wrote:
> > flock02
> >                              mean   variance      sigma        max        min
> >                     tip-1    11.8994     0.5874     0.7664    13.2022     8.6324
> >                     tip-2    11.7394     0.5252     0.7247    13.2540     9.7513
> >                     tip-3    11.8155     0.5288     0.7272    13.2700     9.9480
> >        tip+percpu-rswem-1    15.3601     0.8981     0.9477    16.8116    12.6910
> >        tip+percpu-rswem-2    15.2558     0.8442     0.9188    17.0199    12.9586
> >        tip+percpu-rswem-3    15.5297     0.6386     0.7991    17.4392    12.7992
> 
> I did indeed manage to get flock02 down to a usable level and found:

Aside from the flock_lock_file function moving up, we also get an
increase in _raw_spin_lock.

Before:

     5.17%     5.17%  flock02       [kernel.vmlinux]            [k] _raw_spin_lock
                 |
                 ---_raw_spin_lock
                    |          
                    |--99.75%-- flock_lock_file_wait
                    |          sys_flock
                    |          entry_SYSCALL_64_fastpath
                    |          flock
                     --0.25%-- [...]


After:

     7.20%     7.20%  flock02       [kernel.vmlinux]            [k] _raw_spin_lock
                 |
                 ---_raw_spin_lock
                    |          
                    |--52.23%-- flock_lock_file_wait
                    |          sys_flock
                    |          entry_SYSCALL_64_fastpath
                    |          flock
                    |          
                    |--25.92%-- flock_lock_file
                    |          flock_lock_file_wait
                    |          sys_flock
                    |          entry_SYSCALL_64_fastpath
                    |          flock
                    |          
                    |--21.42%-- locks_delete_lock_ctx
                    |          flock_lock_file
                    |          flock_lock_file_wait
                    |          sys_flock
                    |          entry_SYSCALL_64_fastpath
                    |          flock
                     --0.43%-- [...]


And its not at all clear to me why this would be. It looks like
FILE_LOCK_DEFERRED is happening, but I've not yet figured out why that
would be.
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