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

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Message-ID: <alpine.DEB.2.00.0903211146430.13615@gandalf.stny.rr.com>
Date:	Sat, 21 Mar 2009 11:50:25 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Andrew Morton <akpm@...ux-foundation.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, 21 Mar 2009, Peter Zijlstra wrote:

> 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.

Well, the branch profilers, I would think, are too heavy for 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.

I'd like to get the function graph version working, in order to calculate 
the times spent in each funtion, then I can see how we can (if possible) 
convert it over to perf counters. :-/


> 
> 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.

I need to take a deeper look at perf counters. Not sure why pt_regs would 
be needed for the function counting. All the information needed is passed 
via the mcount call.

-- Steve

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