linux-kernel - Re: [PATCH 0/4] seqlock: Add new blocking reader type & use rwlock

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Date:	Fri, 23 Aug 2013 18:49:54 -0400
From:	Waiman Long <waiman.long@...com>
To:	Alexander Viro <viro@...iv.linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	Waiman Long <Waiman.Long@...com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH 0/4] seqlock: Add new blocking reader type & use rwlock

On 07/03/2013 09:52 PM, Waiman Long wrote:
> During some perf-record sessions of the kernel running the high_systime
> workload of the AIM7 benchmark, it was found that quite a large portion
> of the spinlock contention was due to the perf_event_mmap_event()
> function itself. This perf kernel function calls d_path() which,
> in turn, call path_get() and dput() indirectly. These 3 functions
> were the hottest functions shown in the perf-report output of
> the _raw_spin_lock() function in an 8-socket system with 80 cores
> (hyperthreading off) with a 3.10-rc1 kernel:
>
> -  13.91%           reaim  [kernel.kallsyms]     [k] _raw_spin_lock
>     - _raw_spin_lock
>        + 35.54% path_get
>        + 34.85% dput
>        + 19.49% d_path
>
> In fact, the output of the "perf record -s -a" (without call-graph)
> showed:
>
>   13.37%           reaim  [kernel.kallsyms]     [k] _raw_spin_lock
>    7.61%              ls  [kernel.kallsyms]     [k] _raw_spin_lock
>    3.54%            true  [kernel.kallsyms]     [k] _raw_spin_lock
>
> Without using the perf monitoring tool, the actual execution profile
> will be quite different. In fact, with this patch set and my other
> lockless reference count update patch applied, the output of the same
> "perf record -s -a" command became:
>
>    2.82%           reaim  [kernel.kallsyms]     [k] _raw_spin_lock
>    1.11%              ls  [kernel.kallsyms]     [k] _raw_spin_lock
>    0.26%            true  [kernel.kallsyms]     [k] _raw_spin_lock
>
> So the time spent on _raw_spin_lock() function went down from 24.52%
> to 4.19%. It can be seen that the performance data collected by the
> perf-record command can be heavily skewed in some cases on a system
> with a large number of CPUs. This set of patches enables the perf
> command to give a more accurate and reliable picture of what is really
> happening in the system. At the same time, they can also improve the
> general performance of systems especially those with a large number
> of CPUs.
>
> The d_path() function takes the following two locks:
>
> 1. dentry->d_lock [spinlock] from dget()/dget_parent()/dput()
> 2. rename_lock    [seqlock]  from d_path()
>
> This set of patches address the rename_lock bottleneck by changing the
> way seqlock is implemented so that we can optionally use a read/write
> lock as the underlying implementation instead of the default spinlock.
>
> Incidentally, this patch also provides slight 5% performance boost
> over just the the lockless reference count update patch in the short
> workload of the AIM7 benchmark running on a 8-socket 80-core DL980
> machine on a 3.10-based kernel. There were still a few percentage
> points of contention in d_path() and getcwd syscall left due to their
> use of the rename_lock.
>
> Signed-off-by: Waiman Long<Waiman.Long@...com>
>
> Waiman Long (4):
>    seqlock: Add a new blocking reader type
>    dcache: Use blocking reader seqlock when protected data are not
>      changed
>    seqlock: Allow the use of rwlock in seqlock
>    dcache: Use rwlock as the underlying lock in rename_lock
>
>   fs/dcache.c             |   28 ++++----
>   include/linux/seqlock.h |  167 ++++++++++++++++++++++++++++++++++++++++-------
>   2 files changed, 158 insertions(+), 37 deletions(-)
>

I haven't received any feedback on this patchset. Would you mind letting 
me know if any further change will be needed to make it acceptable to be 
merged?

Thank,
Longman
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