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Date: Sat, 21 Mar 2009 04:26:33 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
       Peter Zijlstra <peterz@infradead.org>,
       Thomas Gleixner <tglx@linutronix.de>,
       =?ISO-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>,
       Arjan van de Ven <arjan@infradead.org>
Subject: Re: [PATCH][GIT PULL] tracing: add function profiler
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On Sat, 21 Mar 2009 00:37:59 -0400 (EDT) Steven Rostedt <rostedt@goodmis.org> wrote:

>    This patch adds a function profiler. In debugfs/tracing/ two new
>     files are created.
>     
>       function_profile_enabled  - to enable or disable profiling
>     
>       trace_stat/functions   - the profiled functions.
>     
>     For example:
>     
>       echo 1 > /debugfs/tracing/function_profile_enabled
>       ./hackbench 50
>       echo 0 > /debugfs/tracing/function_profile_enabled
>     
>     yields:
>     
>       cat /debugfs/tracing/trace_stat/functions
>     
>       Function                               Hit
>       --------                               ---
>       _spin_lock                        10106442
>       _spin_unlock                      10097492
>       kfree                              6013704
>       _spin_unlock_irqrestore            4423941
>       _spin_lock_irqsave                 4406825
>       __phys_addr                        4181686
>       __slab_free                        4038222
>       dput                               4030130
>       path_put                           4023387
>       unroll_tree_refs                   4019532
>     [...]
>     
>     The most hit functions are listed first. Functions that are not
>     hit are not listed.

Why is this useful?

Can we think of any scenarios where kernel developers would get
useful-to-them results from this?  Results which couldn't be 
obtained by other similarly-accessible means?

<strains a bit>

I guess that one could run workload A, look at
/debugfs/tracing/trace_stat/functions changes, then run worklaod B, then
look at its /debugfs/tracing/trace_stat/functions changes, then somehow
glean some information about the differences between the effects of the two
workloads on the kernel.  Or something.

But in this rather fake example and, I suspect, in many others, the result
will be less useful than using oprofile/etc in the same fashion.