linux-kernel - Re: [PATCH][GIT PULL] tracing: add function profiler

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Date:	Sat, 21 Mar 2009 12:57:03 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH][GIT PULL] tracing: add function profiler

On Sat, 2009-03-21 at 04:26 -0700, Andrew Morton wrote:
> On Sat, 21 Mar 2009 00:37:59 -0400 (EDT) Steven Rostedt <rostedt@...dmis.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.

I have to agree with Andrew here, my  plan is to remove all the
profiling stuff from kernel/trace in favour of perf counters.

If you want exact function count profiling we could try to do something
perf counter based, eg. stick a software counter in the mcount thingy.

After that you'd need to get something like
this_pt_regs()/caller_pt_regs() which would provide the current kernel
stack information to generate profile information from.

Current software events use get_irq_regs() ?: task_pt_regs() for lack of
anything better.


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