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

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Date:	Wed, 04 Sep 2013 10:52:51 -0400
From:	Waiman Long <waiman.long@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...nel.org>, Al Viro <viro@...iv.linux.org.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
 update of refcount

On 09/03/2013 03:09 PM, Linus Torvalds wrote:
> On Tue, Sep 3, 2013 at 8:34 AM, Linus Torvalds
> <torvalds@...ux-foundation.org>  wrote:
>> I suspect the tty_ldisc_lock() could be made to go away if we care.
> Heh. I just pulled the tty patches from Greg, and the locking has
> changed completely.
>
> It may actually fix your AIM7 test-case, because while the global
> spinlock remains (it got renamed to "tty_ldiscs_lock" - there's an
> added "s"), the common operations now take the per-tty lock to get the
> ldisc for that tty, rather than that global spinlock (which just
> protects the actual ldisk array now).
>
> That said, I don't know what AIM7 really ends up doing, but your
> profile seems to have every access through tty_ldisc_[de]ref() that
> now uses only the per-tty lock. Of course, how much that helps ends up
> depending on whether AIM7 uses lots of tty's or just one shared one.
>
> Anyway, it might be worth testing my current -git tree.
>
>                    Linus

The latest tty patches did work. The tty related spinlock contention is 
now completely gone. The short workload can now reach over 8M JPM which 
is the highest I have ever seen.

The perf profile was:

5.85%     reaim  reaim                 [.] mul_short
4.87%     reaim  [kernel.kallsyms]     [k] ebitmap_get_bit
4.72%     reaim  reaim                 [.] mul_int
4.71%     reaim  reaim                 [.] mul_long
2.67%     reaim  libc-2.12.so          [.] __random_r
2.64%     reaim  [kernel.kallsyms]     [k] lockref_get_not_zero
1.58%     reaim  [kernel.kallsyms]     [k] copy_user_generic_string
1.48%     reaim  [kernel.kallsyms]     [k] mls_level_isvalid
1.35%     reaim  [kernel.kallsyms]     [k] find_next_bit
1.23%     reaim  [kernel.kallsyms]     [k] system_call
1.21%     reaim  libc-2.12.so          [.] memcpy
1.19%     reaim  [kernel.kallsyms]     [k] _raw_spin_lock
1.06%     reaim  [kernel.kallsyms]     [k] avc_has_perm_flags
1.04%     reaim  libc-2.12.so          [.] __srandom_r
1.02%     reaim  reaim                 [.] newton_raphson
1.01%     reaim  [kernel.kallsyms]     [k] update_cfs_rq_blocked_load
0.98%     reaim  [kernel.kallsyms]     [k] fsnotify
0.94%     reaim  [kernel.kallsyms]     [k] avtab_search_node
0.91%     reaim  libm-2.12.so          [.] __sincos

I have a patch in linux-next that should eliminate ebitmap_get_bit, 
mls_leve_isvalid and find_next_bit from the list top once it is merged.

Regards,
Longman
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