Re: [RFC] [Patch 4/4] lock contention tracking slimmed down

From: Martin Peschke
Date: Mon Jun 11 2007 - 06:32:19 EST

Peter Zijlstra wrote:
On Fri, 2007-06-08 at 19:37 +0200, Martin Peschke wrote:
Peter Zijlstra wrote:
On Fri, 2007-06-08 at 19:18 +0200, Martin Peschke wrote:
Peter Zijlstra wrote:
On Fri, 2007-06-08 at 19:00 +0200, Martin Peschke wrote:
Peter Zijlstra wrote:
I'm confused as to where the class->stat objects are initialised? Is
that done in lock_stat_init()? If so, then you have a bug.
static struct lock_class lock_classes[MAX_LOCKDEP_KEYS];

I assume this gets us class structures containing all zeros, doesn't it?
Then class->stat is zeros as well, which is handled by lib/statistics.
(In this case, data gathering hasn't been turned on yet, and statistic_inc()
and similar functions don't access other areas of struct statistic.)
Who eventually calls percpu_alloc?
There is a small state machine calling percpu_alloc when users do

echo state=on > /debug/statistics/lockdep/definition

So data gathering is off by default.

It might make sense to allow "state=on" as a default. Then allocation would
be done in the context of statistic_attach().
Right, the problem here is that you iterate over all_lock_classes once
at init.

Contrary to what the name might suggest, it are not all possible
classes, just all active ones. So you'll only attach the classes which
have been used up until the init point. All other classes used later
will never be initialized.

One could work around this by iterating over the entire array...

Anyway, this is not good enough. The right solution was to teach my
code to deal with static (per-cpu) statistic counter arrays in addition
to buffers allocated with percpu_alloc(). Both use cases are valid.

Thanks for your feedback.


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