linux-kernel - Re: [PATCH 2/5][RFC] tracing: move function profiler data out of function struct

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Message-ID: <20090325174446.GE14250@elte.hu>
Date:	Wed, 25 Mar 2009 18:44:46 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Tim Bird <tim.bird@...sony.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 2/5][RFC] tracing: move function profiler data out of
	function struct


* Steven Rostedt <rostedt@...dmis.org> wrote:

> 
> On Wed, 25 Mar 2009, Ingo Molnar wrote:
> 
> > 
> > * Steven Rostedt <rostedt@...dmis.org> wrote:
> > 
> > > +/* Interrupts must be disabled calling this */
> > > +static struct ftrace_profile *
> > > +ftrace_profile_alloc(unsigned long ip, bool alloc_safe)
> > > +{
> > > +	struct ftrace_profile *rec = NULL;
> > > +
> > > +	/* prevent recursion */
> > > +	if (atomic_inc_return(&__get_cpu_var(ftrace_profile_disable)) != 1)
> > > +		goto out;
> > > +
> > > +	__raw_spin_lock(&ftrace_profile_rec_lock);
> > > +
> > > +	/* Try to always keep another page available */
> > > +	if (!profile_pages->next && alloc_safe)
> > > +		profile_pages->next = (void *)get_zeroed_page(GFP_ATOMIC);
> > 
> > this does not seem to be NMI safe.
> 
> The atomic_inc_return protects against NMIs, since this is the only place 
> the lock is taken.

i mean, if this code executes _ni_ an NMI. Or that cannot happen? We 
trace nmis too, dont we?

> > This all would be solved much more robustly by the function 
> > attributes hash approach i suggested in the previous mail. If 
> > percpu_alloc() is done for 20,000 functions the memory 
> > allocation overhead is no big deal.
> 
> Later patches create a per-cpu buffers and removes the lock.

ok :)

	Ingo
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