linux-kernel - Re: [PATCH 0/2 v2] dcache: get/release read lock in read_seqbegin_or

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Date:	Thu, 12 Sep 2013 15:01:55 -0400
From:	Waiman Long <waiman.long@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Alexander Viro <viro@...iv.linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH 0/2 v2] dcache: get/release read lock in read_seqbegin_or_lock()
 & friend

On 09/12/2013 01:30 PM, Linus Torvalds wrote:
> On Thu, Sep 12, 2013 at 9:38 AM, Linus Torvalds
> <torvalds@...ux-foundation.org>  wrote:
>> On Thu, Sep 12, 2013 at 7:55 AM, Waiman Long<Waiman.Long@...com>  wrote:
>>> Change log
>>> ----------
>>> v1->v2:
>>>    - Rename the new seqlock primitives to read_seqexcl_lock* and
>>>      read_seqexcl_unlock*.
>> Applied.
> Btw, when I tried to benchmark this, I failed miserably.
>
> Why?

This patch is just a safety guard to prevent occasional bad performance 
because of some bad timing. It will not improve performance for many 
cases because the seqbegin/seqretry sequence succeeds without actual retry.

> If you do a threaded benchmark of "getcwd()", you end up spending all
> your time in a spinlock anyway: get_fs_root_and_pwd() takes the
> fs->lock to get the root/pwd.

I am aware that there is another spinlock bottleneck in the fs struct 
for getcwd().

> Now, AIM7 probably uses processes, not threads, so you don't see this,
> and maybe I shouldn't care. But looking at it, it annoys me
> enormously, because the whole get_fs_root_and_pwd() is just stupid.

AIM7 don't do much getcwd() calls, so it is not a real bottleneck for 
the benchmark. The lockref patch boosts the short workload performance. 
The  prepend_path patch was to fix the incorrect perf record data as 
perf makes heavy use of d_path(). The change made to getcwd() was just a 
side benefit. But then it still have other spinlock bottleneck.

> Putting it all under the RCU lock and then changing it to use
> get_fs_root_and_pwd_rcu() that just uses the fs->seq sequence
> read-lock looks absolutely trivial.

Yes, I think we can do something similar for this. I will take a look to 
see how it can be fixed.

-Longman
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