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

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Date:	Sun, 1 Sep 2013 08:32:34 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Sedat Dilek <sedat.dilek@...il.com>
Cc:	Waiman Long <waiman.long@...com>, Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	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 Sun, Sep 1, 2013 at 3:01 AM, Sedat Dilek <sedat.dilek@...il.com> wrote:
>
> Looks like this is now 10x faster: ~2.66Mloops (debug) VS.
> ~26.60Mloops (no-debug).

Ok, that's getting to be in the right ballpark.

But your profile is still odd.

> Samples: 159K of event 'cycles:pp', Event count (approx.): 76968896763
>  12,79%  t_lockref_from-  [kernel.kallsyms]     [k] irq_return
>   4,36%  t_lockref_from-  [kernel.kallsyms]     [k] __ticket_spin_lock

If you do the profile with "-g", what are the top callers of this? You
shouldn't see any spinlock load from the path lookup, but you have all
these other things going on..

>   4,36%  t_lockref_from-  [kernel.kallsyms]     [k] __acct_update_integrals
>   4,07%  t_lockref_from-  [kernel.kallsyms]     [k] user_exit
>   3,12%  t_lockref_from-  [kernel.kallsyms]     [k] local_clock
>   2,83%  t_lockref_from-  [kernel.kallsyms]     [k] lockref_get_or_lock
>   2,73%  t_lockref_from-  [kernel.kallsyms]     [k] kmem_cache_alloc
>   2,62%  t_lockref_from-  [kernel.kallsyms]     [k] __d_lookup_rcu

You're spending more time on the task stats than on the actual lookup.
Maybe you should turn off CONFIG_TASKSTATS..But why that whole
irq_return thing? Odd.

                   Linus
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