Message-ID: <20081029174001.GA30796@Krystal>
Date:	Wed, 29 Oct 2008 13:40:01 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	ltt-dev@...ts.casi.polymtl.ca,
	"Martin J. Bligh" <mbligh@...igh.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	kernel-trace-list@...hat.com, linux-kernel@...r.kernel.org,
	systemtap@...rces.redhat.com
Subject: Re: [ltt-dev] LTTng 0.44 and LTTV 0.11.3

* Lai Jiangshan (laijs@...fujitsu.com) wrote:
> Mathieu Desnoyers wrote:
> > - I have also vastly simplified locking in the markers and tracepoints
> >   by using _only_ the modules mutex. I actually took this mutex out of
> >   module.c and created its own file so tracepoints and markers can use
> >   it. That should please Lai Jiangshan. Although he may have some work
> >   to do to see how his new probes manager might benefit from it.
> > 
> >   See :
> >   http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=commitdiff;h=7aea87ac46df7613d68034f5904bc8d575069076
> >   and
> >   http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=commitdiff;h=5f6814237f7a67650e7b6214d916825e3f8fc1b7
> >   http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=commitdiff;h=410ba66a1cbe27a611e1c18c0a53e87b4652a2c9
> > 
> 
> Hi, Mathieu,
> 
> I strongly reject for removing tracepoint_mutex and marker_mutex.
> 
> As an independent subsystem, we should use our own locks. Do not use others.
> otherwise coupling will be increased in linux kernel.
> I condemn unnecessary coupling.
> 
> Our tracepoint & marker had tied to modules(for traveling all tracepoints
> or markers). The best thing is that we do not increase the coupling.
> 
> [PATCH 2/2] tracepoint: introduce *_noupdate APIs.
> is helpful for auto-active-tracepoint-mechanism.
> 
> 		Thanx, Lai.
> 

Hi Lai,

The approach you propose looks interesting. Please see below to make
sure we are on the same page.

The problem is that when we want to connect
markers/tracepoints/immediate values together, it results in a real
locking mess between

modules_mutex
  markers_mutex
    tracepoints_mutex
      imv_mutex

When we want to take care of a marker at module load, we have to insure
the following calling scenario is correct :

load_module()
  call markers_update_probes_range() (on the module markers)
    call tracepoint register (to automatically enable a tracepoint
                               when a marker is connected to it)
      call tracepoints_update_probe_range (on kernel core and all modules)
        call imv_update_range (on kernel core and all modules)

The current locking status of tracepoints vs markers does not currently
allow tracepoints_register to be called from the marker update because
it would take the modules_mutex twice.

What you propose is something like this :

load_module()
  call markers_update_probes_range()
    call tracepoint_register_noupdate (to automatically enable a tracepoint
                                       when a marker is connected to it)
  call tracepoints_update_all() (for core kernel and all modules (*))
    name##__imv = (i)
  call imv_update_all() (for core kernel and all modules (*))

(*) This is required because registering a tracepoint might have impact
    outside of the module in which the marker is located. Same for
    changing an immediate value.

And on marker_register_probe() :
  call markers_update_probes_range()
    call tracepoint_register_noupdate
  call tracepoints_update_all()
    name##__imv = (i)
  call imv_update_all()

Which basically uses the same trick I used for immediate values : it
separates the "backing data" update (name##_imv = (i)) from the actual
update that needs to iterate on the modules.

The only thing we have to be aware of is that it actually couples
markers/tracepoints/immediate values much more thightly to keep separate
locking for each, because, as the example above shows, the markers have
to be aware that they must call tracepoints_update_all and
imv_update_all explicitely. On the plus side, it requires much less
iterations on the module sections, which is a clear win.

So the expected mutex nesting order is (indent implies "nested in"):

On load_module :

modules_mutex
  markers_mutex
  tracepoints_mutex
  imv_mutex

On marker register :

markers_mutex
  tracepoints_mutex
  imv_mutex

On tracepoint register :

tracepoints_mutex
  imv_mutex

On imv_update :

imv_mutex

So yes, I think your approach is good, although there are some
implementation quirks in the patch you submitted. I'll comment by
replying to your other post.

Thanks,

Mathieu


> > So hopefully everyone will be happy with this new release. :)
> > 
> > Mathieu
> > 
> 
> 
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-- 
Mathieu Desnoyers
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